diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..e50675e
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,59 @@
+# Byte-compiled / optimized / DLL files
+__pycache__/
+*.py[cod]
+*$py.class
+
+# C extensions
+*.so
+
+# Distribution / packaging
+.Python
+env/
+build/
+develop-eggs/
+dist/
+downloads/
+eggs/
+.eggs/
+lib/
+lib64/
+parts/
+sdist/
+var/
+*.egg-info/
+.installed.cfg
+*.egg
+
+# PyInstaller
+#  Usually these files are written by a python script from a template
+#  before PyInstaller builds the exe, so as to inject date/other infos into it.
+*.manifest
+*.spec
+
+# Installer logs
+pip-log.txt
+pip-delete-this-directory.txt
+
+# Unit test / coverage reports
+htmlcov/
+.tox/
+.coverage
+.coverage.*
+.cache
+nosetests.xml
+coverage.xml
+*,cover
+
+# Translations
+*.mo
+*.pot
+
+# Django stuff:
+*.log
+
+# Sphinx documentation
+docs/_build/
+
+# PyBuilder
+target/
+
diff --git a/README.md b/README.md
index 7a24035..5eb72e3 100644
--- a/README.md
+++ b/README.md
@@ -4,6 +4,158 @@
 
 This repository provides Python APIs for serializing and de-serializing STIX 2 JSON content, along with higher-level APIs for common tasks, including data markings, versioning, and for resolving STIX IDs across multiple data sources.
 
+## Installation
+
+Install with [`pip`](https://pip.pypa.io/en/stable/):
+
+```
+pip install stix2
+```
+
+## Usage
+
+### Creating STIX Domain Objects
+
+To create a STIX object, provide keyword arguments to the type's constructor:
+
+```python
+from stix2 import Indicator
+
+indicator = Indicator(name="File hash for malware variant",
+                      labels=['malicious-activity'],
+                      pattern='file:hashes.md5 = "d41d8cd98f00b204e9800998ecf8427e"')
+
+```
+
+Certain required attributes of all objects will be set automatically if not
+provided as keyword arguments:
+
+- If not provided, `type` will be set automatically to the correct type.
+  You can also provide the type explicitly, but this is not necessary:
+
+  ```python
+  indicator = Indicator(type='indicator', ...)
+  ```
+
+  Passing a value for `type` that does not match the class being constructed
+  will cause an error:
+
+  ```python
+  >>> indicator = Indicator(type='xxx', ...)
+  ValueError: Indicators must have type='indicator'
+  ```
+
+- If not provided, `id` will be generated randomly. If you provide an `id`
+  argument, it must begin with the correct prefix:
+
+  ```python
+  >>> indicator = Indicator(id="campaign--63ce9068-b5ab-47fa-a2cf-a602ea01f21a")
+  ValueError: Indicator id values must begin with 'indicator--'
+  ```
+
+- If not provided, `created` and `modified` will be set to the (same) current
+  time.
+
+For indicators, `labels` and `pattern` are required and cannot be set
+automatically. Trying to create an indicator that is missing one of these fields
+will result in an error:
+
+```python
+>>> indicator = Indicator()
+ValueError: Missing required field(s) for Indicator: (labels, pattern).
+```
+
+However, the required `valid_from` attribute on Indicators will be set to the
+current time if not provided as a keyword argument.
+
+Once created, the object acts like a frozen dictionary. Properties can be
+accessed using the standard Python dictionary syntax:
+
+```python
+>>> indicator['name']
+'File hash for malware variant'
+```
+
+TBD: Should we allow property access using the standard Python attribute syntax?
+
+```python
+>>> indicator.name
+'File hash for malware variant'
+```
+
+Attempting to modify any attributes will raise an error:
+
+```python
+>>>indicator['name'] = "This is a revised name"
+ValueError: Cannot modify properties after creation.
+```
+
+To update the properties of an object, see [Versioning](#versioning) below.
+
+Creating a Malware object follows the same pattern:
+
+```python
+from stix2 import Malware
+
+malware = Malware(name="Poison Ivy",
+                  labels=['remote-access-trojan'])
+```
+
+As with indicators, the `type`, `id`, `created`, and `modified` properties will
+be set automatically if not provided. For Malware objects, the `labels` and
+`name` properties must be provided.
+
+### Creating Relationships
+
+STIX 2 Relationships are separate objects, not properties of the object on
+either side of the relationship. They are constructed similarly to other STIX
+objects. The `type`, `id`, `created`, and `modified` properties are added
+automatically if not provided. Callers must provide the `relationship_type`,
+`source_ref`, and `target_ref` properties.
+
+```python
+from stix2 import Relationship
+
+relationship = Relationship(relationship_type='indicates',
+                            source_ref=indicator.id,
+                            target_ref=malware.id)
+```
+
+The `source_ref` and `target_ref` properties can be either the ID's of other
+STIX objects, or the STIX objects themselves. For readability, Relationship
+objects can also be constructed with the `source_ref`, `relationship_type`, and
+`target_ref` as positional (non-keyword) arguments:
+
+```python
+relationship = Relationship(indicator, 'indicates', malware)
+```
+
+### Creating Bundles
+
+STIX Bundles can be created by passing objects as arguments to the Bundle
+constructor. All required properties (`type`, `id`, and `spec_version`) will be
+set automatically if not provided, or can be provided as keyword arguments:
+
+```python
+from stix2 import bundle
+
+bundle = Bundle(indicator, malware, relationship)
+```
+
+### Serializing STIX objects
+
+The string representation of all STIX classes is a valid STIX JSON object.
+
+```python
+indicator = Indicator(...)
+
+print(str(indicator))
+```
+
+### Versioning
+
+TBD
+
 
 ## Governance
 
diff --git a/requirements.txt b/requirements.txt
new file mode 100644
index 0000000..d10154e
--- /dev/null
+++ b/requirements.txt
@@ -0,0 +1,5 @@
+pytest
+tox
+pytest-cov
+
+-e .
diff --git a/setup.py b/setup.py
new file mode 100644
index 0000000..22ee0ac
--- /dev/null
+++ b/setup.py
@@ -0,0 +1,15 @@
+#!/usr/bin/env python
+from setuptools import setup, find_packages
+
+install_requires = [
+    'pytz',
+]
+
+setup(
+    name='stix2',
+    description="Produce and consume STIX 2 JSON content",
+    version='0.0.1',
+    packages=find_packages(),
+    install_requires=install_requires,
+    keywords="stix stix2 json cti cyber threat intelligence",
+)
diff --git a/stix2/__init__.py b/stix2/__init__.py
new file mode 100644
index 0000000..3991a64
--- /dev/null
+++ b/stix2/__init__.py
@@ -0,0 +1,6 @@
+"""Python APIs for STIX 2."""
+
+from .bundle import Bundle
+from .common import ExternalReference
+from .sdo import Indicator, Malware
+from .sro import Relationship
diff --git a/stix2/base.py b/stix2/base.py
new file mode 100644
index 0000000..f23da74
--- /dev/null
+++ b/stix2/base.py
@@ -0,0 +1,109 @@
+"""Base class for type definitions in the stix2 library."""
+
+import collections
+import datetime as dt
+import json
+import uuid
+
+from .utils import format_datetime, get_timestamp, NOW
+
+__all__ = ['STIXJSONEncoder', '_STIXBase']
+
+DEFAULT_ERROR = "{type} must have {field}='{expected}'."
+
+
+class STIXJSONEncoder(json.JSONEncoder):
+
+    def default(self, obj):
+        if isinstance(obj, (dt.date, dt.datetime)):
+            return format_datetime(obj)
+        elif isinstance(obj, _STIXBase):
+            return dict(obj)
+        else:
+            return super(STIXJSONEncoder, self).default(obj)
+
+
+class _STIXBase(collections.Mapping):
+    """Base class for STIX object types"""
+
+    @classmethod
+    def _make_id(cls):
+        return cls._type + "--" + str(uuid.uuid4())
+
+    def __init__(self, **kwargs):
+        cls = self.__class__
+        class_name = cls.__name__
+
+        # Use the same timestamp for any auto-generated datetimes
+        now = get_timestamp()
+
+        # Detect any keyword arguments not allowed for a specific type
+        extra_kwargs = list(set(kwargs) - set(cls._properties))
+        if extra_kwargs:
+            raise TypeError("unexpected keyword arguments: " + str(extra_kwargs))
+
+        required_fields = [k for k, v in cls._properties.items() if v.get('required')]
+        missing_kwargs = set(required_fields) - set(kwargs)
+        if missing_kwargs:
+            msg = "Missing required field(s) for {type}: ({fields})."
+            field_list = ", ".join(x for x in sorted(list(missing_kwargs)))
+            raise ValueError(msg.format(type=class_name, fields=field_list))
+
+        for prop_name, prop_metadata in cls._properties.items():
+            if prop_name not in kwargs:
+                if prop_metadata.get('default'):
+                    default = prop_metadata['default']
+                    if default == NOW:
+                        kwargs[prop_name] = now
+                    else:
+                        kwargs[prop_name] = default(cls)
+                elif prop_metadata.get('fixed'):
+                    kwargs[prop_name] = prop_metadata['fixed']
+
+            if prop_metadata.get('validate'):
+                if not prop_metadata['validate'](cls, kwargs[prop_name]):
+                    msg = prop_metadata.get('error_msg', DEFAULT_ERROR).format(
+                        type=class_name,
+                        field=prop_name,
+                        expected=prop_metadata.get('expected',
+                                                   prop_metadata['default'])(cls),
+                    )
+                    raise ValueError(msg)
+            elif prop_metadata.get('fixed'):
+                if kwargs[prop_name] != prop_metadata['fixed']:
+                    msg = prop_metadata.get('error_msg', DEFAULT_ERROR).format(
+                        type=class_name,
+                        field=prop_name,
+                        expected=prop_metadata['fixed']
+                    )
+                    raise ValueError(msg)
+
+        self._inner = kwargs
+
+    def __getitem__(self, key):
+        return self._inner[key]
+
+    def __iter__(self):
+        return iter(self._inner)
+
+    def __len__(self):
+        return len(self._inner)
+
+    # Handle attribute access just like key access
+    def __getattr__(self, name):
+        return self.get(name)
+
+    def __setattr__(self, name, value):
+        if name != '_inner':
+            raise ValueError("Cannot modify properties after creation.")
+        super(_STIXBase, self).__setattr__(name, value)
+
+    def __str__(self):
+        # TODO: put keys in specific order. Probably need custom JSON encoder.
+        return json.dumps(self, indent=4, sort_keys=True, cls=STIXJSONEncoder,
+                          separators=(",", ": "))  # Don't include spaces after commas.
+
+    def __repr__(self):
+        props = [(k, self[k]) for k in sorted(self._properties) if self.get(k)]
+        return "{0}({1})".format(self.__class__.__name__,
+                                 ", ".join(["{0!s}={1!r}".format(k, v) for k, v in props]))
diff --git a/stix2/bundle.py b/stix2/bundle.py
new file mode 100644
index 0000000..614ce0d
--- /dev/null
+++ b/stix2/bundle.py
@@ -0,0 +1,24 @@
+"""STIX 2 Bundle object"""
+
+from .base import _STIXBase
+from .common import TYPE_PROPERTY, ID_PROPERTY
+
+
+class Bundle(_STIXBase):
+
+    _type = 'bundle'
+    _properties = {
+        'type': TYPE_PROPERTY,
+        'id': ID_PROPERTY,
+        'spec_version': {
+            'fixed': "2.0",
+        },
+        'objects': {},
+    }
+
+    def __init__(self, *args, **kwargs):
+        # Add any positional arguments to the 'objects' kwarg.
+        if args:
+            kwargs['objects'] = kwargs.get('objects', []) + list(args)
+
+        super(Bundle, self).__init__(**kwargs)
diff --git a/stix2/common.py b/stix2/common.py
new file mode 100644
index 0000000..924210a
--- /dev/null
+++ b/stix2/common.py
@@ -0,0 +1,38 @@
+"""STIX 2 Common Data Types and Properties"""
+
+from .base import _STIXBase
+from .utils import NOW
+
+TYPE_PROPERTY = {
+    'default': (lambda x: x._type),
+    'validate': (lambda x, val: val == x._type)
+}
+
+ID_PROPERTY = {
+    'default': (lambda x: x._make_id()),
+    'validate': (lambda x, val: val.startswith(x._type + "--")),
+    'expected': (lambda x: x._type + "--"),
+    'error_msg': "{type} {field} values must begin with '{expected}'."
+}
+
+COMMON_PROPERTIES = {
+    'type': TYPE_PROPERTY,
+    'id': ID_PROPERTY,
+    'created': {
+        'default': NOW,
+    },
+    'modified': {
+        'default': NOW,
+    },
+}
+
+
+class ExternalReference(_STIXBase):
+    _properties = {
+        'source_name': {
+            'required': True,
+        },
+        'description': {},
+        'url': {},
+        'external_id': {},
+    }
diff --git a/stix2/sdo.py b/stix2/sdo.py
new file mode 100644
index 0000000..3a033fe
--- /dev/null
+++ b/stix2/sdo.py
@@ -0,0 +1,64 @@
+"""STIX 2.0 Domain Objects"""
+
+from .base import _STIXBase
+from .common import COMMON_PROPERTIES
+from .utils import NOW
+
+
+class Indicator(_STIXBase):
+
+    _type = 'indicator'
+    _properties = COMMON_PROPERTIES.copy()
+    _properties.update({
+        'labels': {
+            'required': True,
+        },
+        'pattern': {
+            'required': True,
+        },
+        'valid_from': {
+            'default': NOW,
+        },
+    })
+
+    def __init__(self, **kwargs):
+        # TODO:
+        # - created_by_ref
+        # - revoked
+        # - external_references
+        # - object_marking_refs
+        # - granular_markings
+
+        # - name
+        # - description
+        # - valid_until
+        # - kill_chain_phases
+
+        super(Indicator, self).__init__(**kwargs)
+
+
+class Malware(_STIXBase):
+
+    _type = 'malware'
+    _properties = COMMON_PROPERTIES.copy()
+    _properties.update({
+        'labels': {
+            'required': True,
+        },
+        'name': {
+            'required': True,
+        },
+    })
+
+    def __init__(self, **kwargs):
+        # TODO:
+        # - created_by_ref
+        # - revoked
+        # - external_references
+        # - object_marking_refs
+        # - granular_markings
+
+        # - description
+        # - kill_chain_phases
+
+        super(Malware, self).__init__(**kwargs)
diff --git a/stix2/sro.py b/stix2/sro.py
new file mode 100644
index 0000000..856cf3b
--- /dev/null
+++ b/stix2/sro.py
@@ -0,0 +1,51 @@
+"""STIX 2.0 Relationship Objects."""
+
+from .base import _STIXBase
+from .common import COMMON_PROPERTIES
+
+
+class Relationship(_STIXBase):
+
+    _type = 'relationship'
+    _properties = COMMON_PROPERTIES.copy()
+    _properties.update({
+        'relationship_type': {
+            'required': True,
+        },
+        'source_ref': {
+            'required': True,
+        },
+        'target_ref': {
+            'required': True,
+        },
+    })
+
+    # Explicitly define the first three kwargs to make readable Relationship declarations.
+    def __init__(self, source_ref=None, relationship_type=None, target_ref=None,
+                 **kwargs):
+        # TODO:
+        # - created_by_ref
+        # - revoked
+        # - external_references
+        # - object_marking_refs
+        # - granular_markings
+
+        # - description
+
+        # Allow (source_ref, relationship_type, target_ref) as positional args.
+        if source_ref and not kwargs.get('source_ref'):
+            kwargs['source_ref'] = source_ref
+        if relationship_type and not kwargs.get('relationship_type'):
+            kwargs['relationship_type'] = relationship_type
+        if target_ref and not kwargs.get('target_ref'):
+            kwargs['target_ref'] = target_ref
+
+        # If actual STIX objects (vs. just the IDs) are passed in, extract the
+        # ID values to use in the Relationship object.
+        if kwargs.get('source_ref') and isinstance(kwargs['source_ref'], _STIXBase):
+            kwargs['source_ref'] = kwargs['source_ref'].id
+
+        if kwargs.get('target_ref') and isinstance(kwargs['target_ref'], _STIXBase):
+            kwargs['target_ref'] = kwargs['target_ref'].id
+
+        super(Relationship, self).__init__(**kwargs)
diff --git a/stix2/test/__init__.py b/stix2/test/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/stix2/test/test_external_reference.py b/stix2/test/test_external_reference.py
new file mode 100644
index 0000000..353984a
--- /dev/null
+++ b/stix2/test/test_external_reference.py
@@ -0,0 +1,112 @@
+"""Tests for stix.ExternalReference"""
+
+import pytest
+
+import stix2
+
+VERIS = """{
+    "external_id": "0001AA7F-C601-424A-B2B8-BE6C9F5164E7",
+    "source_name": "veris",
+    "url": "https://github.com/vz-risk/VCDB/blob/master/data/json/0001AA7F-C601-424A-B2B8-BE6C9F5164E7.json"
+}"""
+
+
+def test_external_reference_veris():
+    ref = stix2.ExternalReference(
+        source_name="veris",
+        external_id="0001AA7F-C601-424A-B2B8-BE6C9F5164E7",
+        url="https://github.com/vz-risk/VCDB/blob/master/data/json/0001AA7F-C601-424A-B2B8-BE6C9F5164E7.json",
+    )
+
+    assert str(ref) == VERIS
+
+
+CAPEC = """{
+    "external_id": "CAPEC-550",
+    "source_name": "capec"
+}"""
+
+
+def test_external_reference_capec():
+    ref = stix2.ExternalReference(
+        source_name="capec",
+        external_id="CAPEC-550",
+    )
+
+    assert str(ref) == CAPEC
+    assert repr(ref) == "ExternalReference(external_id='CAPEC-550', source_name='capec')"
+
+
+CAPEC_URL = """{
+    "external_id": "CAPEC-550",
+    "source_name": "capec",
+    "url": "http://capec.mitre.org/data/definitions/550.html"
+}"""
+
+
+def test_external_reference_capec_url():
+    ref = stix2.ExternalReference(
+        source_name="capec",
+        external_id="CAPEC-550",
+        url="http://capec.mitre.org/data/definitions/550.html",
+    )
+
+    assert str(ref) == CAPEC_URL
+
+
+THREAT_REPORT = """{
+    "description": "Threat report",
+    "source_name": "ACME Threat Intel",
+    "url": "http://www.example.com/threat-report.pdf"
+}"""
+
+
+def test_external_reference_threat_report():
+    ref = stix2.ExternalReference(
+        source_name="ACME Threat Intel",
+        description="Threat report",
+        url="http://www.example.com/threat-report.pdf",
+    )
+
+    assert str(ref) == THREAT_REPORT
+
+
+BUGZILLA = """{
+    "external_id": "1370",
+    "source_name": "ACME Bugzilla",
+    "url": "https://www.example.com/bugs/1370"
+}"""
+
+
+def test_external_reference_bugzilla():
+    ref = stix2.ExternalReference(
+        source_name="ACME Bugzilla",
+        external_id="1370",
+        url="https://www.example.com/bugs/1370",
+    )
+
+    assert str(ref) == BUGZILLA
+
+
+OFFLINE = """{
+    "description": "Threat report",
+    "source_name": "ACME Threat Intel"
+}"""
+
+
+def test_external_reference_offline():
+    ref = stix2.ExternalReference(
+        source_name="ACME Threat Intel",
+        description="Threat report",
+    )
+
+    assert str(ref) == OFFLINE
+    assert repr(ref) == "ExternalReference(description='Threat report', source_name='ACME Threat Intel')"
+    # Yikes! This works
+    assert eval("stix2." + repr(ref)) == ref
+
+
+def test_external_reference_source_required():
+    with pytest.raises(ValueError) as excinfo:
+        ref = stix2.ExternalReference()
+    assert str(excinfo.value) == "Missing required field(s) for ExternalReference: (source_name)."
diff --git a/stix2/test/test_stix2.py b/stix2/test/test_stix2.py
new file mode 100644
index 0000000..164aca9
--- /dev/null
+++ b/stix2/test/test_stix2.py
@@ -0,0 +1,473 @@
+"""Tests for the stix2 library"""
+
+import datetime as dt
+import uuid
+
+import pytest
+import pytz
+
+import stix2
+import stix2.utils
+
+
+FAKE_TIME = dt.datetime(2017, 1, 1, 12, 34, 56, tzinfo=pytz.utc)
+
+
+# Inspired by: http://stackoverflow.com/a/24006251
+@pytest.fixture
+def clock(monkeypatch):
+
+    class mydatetime(dt.datetime):
+        @classmethod
+        def now(cls, tz=None):
+            return FAKE_TIME
+
+    monkeypatch.setattr(dt, 'datetime', mydatetime)
+
+
+def test_clock(clock):
+    assert dt.datetime.now() == FAKE_TIME
+
+
+@pytest.fixture
+def uuid4(monkeypatch):
+    def wrapper():
+        data = [0]
+
+        def wrapped():
+            data[0] += 1
+            return "00000000-0000-0000-0000-00000000%04x" % data[0]
+
+        return wrapped
+    monkeypatch.setattr(uuid, "uuid4", wrapper())
+
+
+def test_my_uuid4_fixture(uuid4):
+    assert uuid.uuid4() == "00000000-0000-0000-0000-000000000001"
+    assert uuid.uuid4() == "00000000-0000-0000-0000-000000000002"
+    assert uuid.uuid4() == "00000000-0000-0000-0000-000000000003"
+    for _ in range(256):
+        uuid.uuid4()
+    assert uuid.uuid4() == "00000000-0000-0000-0000-000000000104"
+
+
+INDICATOR_ID = "indicator--01234567-89ab-cdef-0123-456789abcdef"
+MALWARE_ID = "malware--fedcba98-7654-3210-fedc-ba9876543210"
+RELATIONSHIP_ID = "relationship--00000000-1111-2222-3333-444444444444"
+
+# Minimum required args for an Indicator instance
+INDICATOR_KWARGS = dict(
+    labels=['malicious-activity'],
+    pattern="[file:hashes.MD5 = 'd41d8cd98f00b204e9800998ecf8427e']",
+)
+
+# Minimum required args for a Malware instance
+MALWARE_KWARGS = dict(
+    labels=['ransomware'],
+    name="Cryptolocker",
+)
+
+# Minimum required args for a Relationship instance
+RELATIONSHIP_KWARGS = dict(
+    relationship_type="indicates",
+    source_ref=INDICATOR_ID,
+    target_ref=MALWARE_ID,
+)
+
+
+@pytest.fixture
+def indicator(uuid4, clock):
+    return stix2.Indicator(**INDICATOR_KWARGS)
+
+
+@pytest.fixture
+def malware(uuid4, clock):
+    return stix2.Malware(**MALWARE_KWARGS)
+
+
+@pytest.fixture
+def relationship(uuid4, clock):
+    return stix2.Relationship(**RELATIONSHIP_KWARGS)
+
+
+EXPECTED_INDICATOR = """{
+    "created": "2017-01-01T00:00:01Z",
+    "id": "indicator--01234567-89ab-cdef-0123-456789abcdef",
+    "labels": [
+        "malicious-activity"
+    ],
+    "modified": "2017-01-01T00:00:01Z",
+    "pattern": "[file:hashes.MD5 = 'd41d8cd98f00b204e9800998ecf8427e']",
+    "type": "indicator",
+    "valid_from": "1970-01-01T00:00:01Z"
+}"""
+
+EXPECTED_INDICATOR_REPR = "Indicator(" + " ".join("""
+    created=datetime.datetime(2017, 1, 1, 0, 0, 1, tzinfo=<UTC>),
+    id='indicator--01234567-89ab-cdef-0123-456789abcdef',
+    labels=['malicious-activity'],
+    modified=datetime.datetime(2017, 1, 1, 0, 0, 1, tzinfo=<UTC>),
+    pattern="[file:hashes.MD5 = 'd41d8cd98f00b204e9800998ecf8427e']",
+    type='indicator',
+    valid_from=datetime.datetime(1970, 1, 1, 0, 0, 1, tzinfo=<UTC>)
+""".split()) + ")"
+
+
+def test_indicator_with_all_required_fields():
+    now = dt.datetime(2017, 1, 1, 0, 0, 1, tzinfo=pytz.utc)
+    epoch = dt.datetime(1970, 1, 1, 0, 0, 1, tzinfo=pytz.utc)
+
+    indicator = stix2.Indicator(
+        type="indicator",
+        id=INDICATOR_ID,
+        labels=['malicious-activity'],
+        pattern="[file:hashes.MD5 = 'd41d8cd98f00b204e9800998ecf8427e']",
+        created=now,
+        modified=now,
+        valid_from=epoch,
+    )
+
+    assert str(indicator) == EXPECTED_INDICATOR
+    assert repr(indicator) == EXPECTED_INDICATOR_REPR
+
+
+def test_indicator_autogenerated_fields(indicator):
+    assert indicator.type == 'indicator'
+    assert indicator.id == 'indicator--00000000-0000-0000-0000-000000000001'
+    assert indicator.created == FAKE_TIME
+    assert indicator.modified == FAKE_TIME
+    assert indicator.labels == ['malicious-activity']
+    assert indicator.pattern == "[file:hashes.MD5 = 'd41d8cd98f00b204e9800998ecf8427e']"
+    assert indicator.valid_from == FAKE_TIME
+
+    assert indicator['type'] == 'indicator'
+    assert indicator['id'] == 'indicator--00000000-0000-0000-0000-000000000001'
+    assert indicator['created'] == FAKE_TIME
+    assert indicator['modified'] == FAKE_TIME
+    assert indicator['labels'] == ['malicious-activity']
+    assert indicator['pattern'] == "[file:hashes.MD5 = 'd41d8cd98f00b204e9800998ecf8427e']"
+    assert indicator['valid_from'] == FAKE_TIME
+
+
+def test_indicator_type_must_be_indicator():
+    with pytest.raises(ValueError) as excinfo:
+        indicator = stix2.Indicator(type='xxx', **INDICATOR_KWARGS)
+
+    assert str(excinfo.value) == "Indicator must have type='indicator'."
+
+
+def test_indicator_id_must_start_with_indicator():
+    with pytest.raises(ValueError) as excinfo:
+        indicator = stix2.Indicator(id='my-prefix--', **INDICATOR_KWARGS)
+
+    assert str(excinfo.value) == "Indicator id values must begin with 'indicator--'."
+
+
+def test_indicator_required_fields():
+    with pytest.raises(ValueError) as excinfo:
+        indicator = stix2.Indicator()
+    assert str(excinfo.value) == "Missing required field(s) for Indicator: (labels, pattern)."
+
+
+def test_indicator_required_field_pattern():
+    with pytest.raises(ValueError) as excinfo:
+        indicator = stix2.Indicator(labels=['malicious-activity'])
+    assert str(excinfo.value) == "Missing required field(s) for Indicator: (pattern)."
+
+
+def test_cannot_assign_to_indicator_attributes(indicator):
+    with pytest.raises(ValueError) as excinfo:
+        indicator.valid_from = dt.datetime.now()
+
+    assert str(excinfo.value) == "Cannot modify properties after creation."
+
+
+def test_invalid_kwarg_to_indicator():
+    with pytest.raises(TypeError) as excinfo:
+        indicator = stix2.Indicator(my_custom_property="foo", **INDICATOR_KWARGS)
+    assert str(excinfo.value) == "unexpected keyword arguments: ['my_custom_property']"
+
+
+def test_created_modified_time_are_identical_by_default():
+    """By default, the created and modified times should be the same."""
+    indicator = stix2.Indicator(**INDICATOR_KWARGS)
+
+    assert indicator.created == indicator.modified
+
+
+EXPECTED_MALWARE = """{
+    "created": "2016-05-12T08:17:27Z",
+    "id": "malware--fedcba98-7654-3210-fedc-ba9876543210",
+    "labels": [
+        "ransomware"
+    ],
+    "modified": "2016-05-12T08:17:27Z",
+    "name": "Cryptolocker",
+    "type": "malware"
+}"""
+
+
+def test_malware_with_all_required_fields():
+    now = dt.datetime(2016, 5, 12, 8, 17, 27, tzinfo=pytz.utc)
+
+    malware = stix2.Malware(
+        type="malware",
+        id=MALWARE_ID,
+        created=now,
+        modified=now,
+        labels=["ransomware"],
+        name="Cryptolocker",
+    )
+
+    assert str(malware) == EXPECTED_MALWARE
+
+
+def test_malware_autogenerated_fields(malware):
+    assert malware.type == 'malware'
+    assert malware.id == 'malware--00000000-0000-0000-0000-000000000001'
+    assert malware.created == FAKE_TIME
+    assert malware.modified == FAKE_TIME
+    assert malware.labels == ['ransomware']
+    assert malware.name == "Cryptolocker"
+
+    assert malware['type'] == 'malware'
+    assert malware['id'] == 'malware--00000000-0000-0000-0000-000000000001'
+    assert malware['created'] == FAKE_TIME
+    assert malware['modified'] == FAKE_TIME
+    assert malware['labels'] == ['ransomware']
+    assert malware['name'] == "Cryptolocker"
+
+
+def test_malware_type_must_be_malware():
+    with pytest.raises(ValueError) as excinfo:
+        malware = stix2.Malware(type='xxx', **MALWARE_KWARGS)
+
+    assert str(excinfo.value) == "Malware must have type='malware'."
+
+
+def test_malware_id_must_start_with_malware():
+    with pytest.raises(ValueError) as excinfo:
+        malware = stix2.Malware(id='my-prefix--', **MALWARE_KWARGS)
+
+    assert str(excinfo.value) == "Malware id values must begin with 'malware--'."
+
+
+def test_malware_required_fields():
+    with pytest.raises(ValueError) as excinfo:
+        malware = stix2.Malware()
+    assert str(excinfo.value) == "Missing required field(s) for Malware: (labels, name)."
+
+
+def test_malware_required_field_name():
+    with pytest.raises(ValueError) as excinfo:
+        malware = stix2.Malware(labels=['ransomware'])
+    assert str(excinfo.value) == "Missing required field(s) for Malware: (name)."
+
+
+def test_cannot_assign_to_malware_attributes(malware):
+    with pytest.raises(ValueError) as excinfo:
+        malware.name = "Cryptolocker II"
+
+    assert str(excinfo.value) == "Cannot modify properties after creation."
+
+
+def test_invalid_kwarg_to_malware():
+    with pytest.raises(TypeError) as excinfo:
+        malware = stix2.Malware(my_custom_property="foo", **MALWARE_KWARGS)
+    assert str(excinfo.value) == "unexpected keyword arguments: ['my_custom_property']"
+
+
+EXPECTED_RELATIONSHIP = """{
+    "created": "2016-04-06T20:06:37Z",
+    "id": "relationship--00000000-1111-2222-3333-444444444444",
+    "modified": "2016-04-06T20:06:37Z",
+    "relationship_type": "indicates",
+    "source_ref": "indicator--01234567-89ab-cdef-0123-456789abcdef",
+    "target_ref": "malware--fedcba98-7654-3210-fedc-ba9876543210",
+    "type": "relationship"
+}"""
+
+
+def test_relationship_all_required_fields():
+    now = dt.datetime(2016, 4, 6, 20, 6, 37, tzinfo=pytz.utc)
+
+    relationship = stix2.Relationship(
+        type='relationship',
+        id=RELATIONSHIP_ID,
+        created=now,
+        modified=now,
+        relationship_type='indicates',
+        source_ref=INDICATOR_ID,
+        target_ref=MALWARE_ID,
+    )
+    assert str(relationship) == EXPECTED_RELATIONSHIP
+
+
+def test_relationship_autogenerated_fields(relationship):
+    assert relationship.type == 'relationship'
+    assert relationship.id == 'relationship--00000000-0000-0000-0000-000000000001'
+    assert relationship.created == FAKE_TIME
+    assert relationship.modified == FAKE_TIME
+    assert relationship.relationship_type == 'indicates'
+    assert relationship.source_ref == INDICATOR_ID
+    assert relationship.target_ref == MALWARE_ID
+
+    assert relationship['type'] == 'relationship'
+    assert relationship['id'] == 'relationship--00000000-0000-0000-0000-000000000001'
+    assert relationship['created'] == FAKE_TIME
+    assert relationship['modified'] == FAKE_TIME
+    assert relationship['relationship_type'] == 'indicates'
+    assert relationship['source_ref'] == INDICATOR_ID
+    assert relationship['target_ref'] == MALWARE_ID
+
+
+def test_relationship_type_must_be_relationship():
+    with pytest.raises(ValueError) as excinfo:
+        relationship = stix2.Relationship(type='xxx', **RELATIONSHIP_KWARGS)
+
+    assert str(excinfo.value) == "Relationship must have type='relationship'."
+
+
+def test_relationship_id_must_start_with_relationship():
+    with pytest.raises(ValueError) as excinfo:
+        relationship = stix2.Relationship(id='my-prefix--', **RELATIONSHIP_KWARGS)
+
+    assert str(excinfo.value) == "Relationship id values must begin with 'relationship--'."
+
+
+def test_relationship_required_field_relationship_type():
+    with pytest.raises(ValueError) as excinfo:
+        relationship = stix2.Relationship()
+    assert str(excinfo.value) == "Missing required field(s) for Relationship: (relationship_type, source_ref, target_ref)."
+
+
+def test_relationship_missing_some_required_fields():
+    with pytest.raises(ValueError) as excinfo:
+        # relationship_type is checked first, so make sure that is provided
+        relationship = stix2.Relationship(relationship_type='indicates')
+    assert str(excinfo.value) == "Missing required field(s) for Relationship: (source_ref, target_ref)."
+
+
+def test_relationship_required_field_target_ref():
+    with pytest.raises(ValueError) as excinfo:
+        relationship = stix2.Relationship(
+            relationship_type='indicates',
+            source_ref=INDICATOR_ID
+        )
+    assert str(excinfo.value) == "Missing required field(s) for Relationship: (target_ref)."
+
+
+def test_cannot_assign_to_relationship_attributes(relationship):
+    with pytest.raises(ValueError) as excinfo:
+        relationship.relationship_type = "derived-from"
+
+    assert str(excinfo.value) == "Cannot modify properties after creation."
+
+
+def test_invalid_kwarg_to_relationship():
+    with pytest.raises(TypeError) as excinfo:
+        relationship = stix2.Relationship(my_custom_property="foo", **RELATIONSHIP_KWARGS)
+    assert str(excinfo.value) == "unexpected keyword arguments: ['my_custom_property']" in str(excinfo)
+
+
+def test_create_relationship_from_objects_rather_than_ids(indicator, malware):
+    relationship = stix2.Relationship(
+        relationship_type="indicates",
+        source_ref=indicator,
+        target_ref=malware,
+    )
+
+    assert relationship.relationship_type == 'indicates'
+    assert relationship.source_ref == 'indicator--00000000-0000-0000-0000-000000000001'
+    assert relationship.target_ref == 'malware--00000000-0000-0000-0000-000000000002'
+    assert relationship.id == 'relationship--00000000-0000-0000-0000-000000000003'
+
+
+def test_create_relationship_with_positional_args(indicator, malware):
+    relationship = stix2.Relationship(indicator, 'indicates', malware)
+
+    assert relationship.relationship_type == 'indicates'
+    assert relationship.source_ref == 'indicator--00000000-0000-0000-0000-000000000001'
+    assert relationship.target_ref == 'malware--00000000-0000-0000-0000-000000000002'
+    assert relationship.id == 'relationship--00000000-0000-0000-0000-000000000003'
+
+
+EXPECTED_BUNDLE = """{
+    "id": "bundle--00000000-0000-0000-0000-000000000004",
+    "objects": [
+        {
+            "created": "2017-01-01T12:34:56Z",
+            "id": "indicator--00000000-0000-0000-0000-000000000001",
+            "labels": [
+                "malicious-activity"
+            ],
+            "modified": "2017-01-01T12:34:56Z",
+            "pattern": "[file:hashes.MD5 = 'd41d8cd98f00b204e9800998ecf8427e']",
+            "type": "indicator",
+            "valid_from": "2017-01-01T12:34:56Z"
+        },
+        {
+            "created": "2017-01-01T12:34:56Z",
+            "id": "malware--00000000-0000-0000-0000-000000000002",
+            "labels": [
+                "ransomware"
+            ],
+            "modified": "2017-01-01T12:34:56Z",
+            "name": "Cryptolocker",
+            "type": "malware"
+        },
+        {
+            "created": "2017-01-01T12:34:56Z",
+            "id": "relationship--00000000-0000-0000-0000-000000000003",
+            "modified": "2017-01-01T12:34:56Z",
+            "relationship_type": "indicates",
+            "source_ref": "indicator--01234567-89ab-cdef-0123-456789abcdef",
+            "target_ref": "malware--fedcba98-7654-3210-fedc-ba9876543210",
+            "type": "relationship"
+        }
+    ],
+    "spec_version": "2.0",
+    "type": "bundle"
+}"""
+
+
+def test_empty_bundle():
+    bundle = stix2.Bundle()
+
+    assert bundle.type == "bundle"
+    assert bundle.id.startswith("bundle--")
+    assert bundle.spec_version == "2.0"
+    assert bundle.objects is None
+
+
+def test_bundle_with_wrong_type():
+    with pytest.raises(ValueError) as excinfo:
+        bundle = stix2.Bundle(type="not-a-bundle")
+
+    assert str(excinfo.value) == "Bundle must have type='bundle'."
+
+
+def test_bundle_id_must_start_with_bundle():
+    with pytest.raises(ValueError) as excinfo:
+        bundle = stix2.Bundle(id='my-prefix--')
+
+    assert str(excinfo.value) == "Bundle id values must begin with 'bundle--'."
+
+
+def test_bundle_with_wrong_spec_version():
+    with pytest.raises(ValueError) as excinfo:
+        bundle = stix2.Bundle(spec_version="1.2")
+
+    assert str(excinfo.value) == "Bundle must have spec_version='2.0'."
+
+
+def test_create_bundle(indicator, malware, relationship):
+    bundle = stix2.Bundle(objects=[indicator, malware, relationship])
+
+    assert str(bundle) == EXPECTED_BUNDLE
+
+
+def test_create_bundle_with_positional_args(indicator, malware, relationship):
+    bundle = stix2.Bundle(indicator, malware, relationship)
+
+    assert str(bundle) == EXPECTED_BUNDLE
diff --git a/stix2/test/test_utils.py b/stix2/test/test_utils.py
new file mode 100644
index 0000000..3eee491
--- /dev/null
+++ b/stix2/test/test_utils.py
@@ -0,0 +1,19 @@
+import datetime as dt
+
+import pytest
+import pytz
+
+import stix2.utils
+
+amsterdam = pytz.timezone('Europe/Amsterdam')
+eastern = pytz.timezone('US/Eastern')
+
+
+@pytest.mark.parametrize('dttm,timestamp', [
+    (dt.datetime(2017, 1, 1, tzinfo=pytz.utc), '2017-01-01T00:00:00Z'),
+    (amsterdam.localize(dt.datetime(2017, 1, 1)), '2016-12-31T23:00:00Z'),
+    (eastern.localize(dt.datetime(2017, 1, 1, 12, 34, 56)), '2017-01-01T17:34:56Z'),
+    (eastern.localize(dt.datetime(2017, 7, 1)), '2017-07-01T04:00:00Z'),
+])
+def test_timestamp_formatting(dttm, timestamp):
+    assert stix2.utils.format_datetime(dttm) == timestamp
diff --git a/stix2/utils.py b/stix2/utils.py
new file mode 100644
index 0000000..0acd5de
--- /dev/null
+++ b/stix2/utils.py
@@ -0,0 +1,26 @@
+"""Utility functions and classes for the stix2 library."""
+
+import datetime as dt
+
+import pytz
+
+# Sentinel value for fields that should be set to the current time.
+# We can't use the standard 'default' approach, since if there are multiple
+# timestamps in a single object, the timestamps will vary by a few microseconds.
+NOW = object()
+
+
+def get_timestamp():
+    return dt.datetime.now(tz=pytz.UTC)
+
+
+def format_datetime(dttm):
+    # TODO: how to handle naive datetime
+
+    # 1. Convert to UTC
+    # 2. Format in ISO format
+    # 3. Strip off "+00:00"
+    # 4. Add "Z"
+
+    # TODO: how to handle timestamps with subsecond 0's
+    return dttm.astimezone(pytz.utc).isoformat()[:-6] + "Z"
diff --git a/tox.ini b/tox.ini
new file mode 100644
index 0000000..70e820e
--- /dev/null
+++ b/tox.ini
@@ -0,0 +1,23 @@
+[tox]
+envlist = py26,py27,py33,py34,py35,py36,pycodestyle
+
+[testenv]
+deps = pytest
+commands = pytest
+
+[testenv:pycodestyle]
+deps = pycodestyle
+commands = pycodestyle ./stix2
+
+[pycodestyle]
+ignore=
+max-line-length=160
+
+[travis]
+python =
+  2.6: py26
+  2.7: py27, pycodestyle
+  3.3: py33
+  3.4: py34
+  3.5: py35
+  3.6: py36