diff --git a/stix2/base.py b/stix2/base.py
index cca5a1f..00caa84 100644
--- a/stix2/base.py
+++ b/stix2/base.py
@@ -23,6 +23,16 @@ class STIXJSONEncoder(json.JSONEncoder):
             return super(STIXJSONEncoder, self).default(obj)
 
 
+def get_required_properties(properties):
+    for k, v in properties.items():
+        if isinstance(v, dict):
+            if v.get('required'):
+                yield k
+        else:  # This is a Property subclass
+            if v.required:
+                yield k
+
+
 class _STIXBase(collections.Mapping):
     """Base class for STIX object types"""
 
@@ -30,19 +40,53 @@ class _STIXBase(collections.Mapping):
     def _make_id(cls):
         return cls._type + "--" + str(uuid.uuid4())
 
+    # TODO: remove this
+    def _handle_old_style_property(self, prop_name, prop_metadata, kwargs):
+        cls = self.__class__
+        class_name = cls.__name__
+
+        if prop_name not in kwargs:
+            if prop_metadata.get('default'):
+                default = prop_metadata['default']
+                if default == NOW:
+                    kwargs[prop_name] = self.__now
+                else:
+                    kwargs[prop_name] = default(cls)
+            elif prop_metadata.get('fixed'):
+                kwargs[prop_name] = prop_metadata['fixed']
+
+        if prop_metadata.get('validate'):
+            if (prop_name in kwargs and
+                    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.get('default', lambda x: ''))(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)
+
     def __init__(self, **kwargs):
         cls = self.__class__
         class_name = cls.__name__
 
         # Use the same timestamp for any auto-generated datetimes
-        now = get_timestamp()
+        self.__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')]
+        required_fields = get_required_properties(cls._properties)
         missing_kwargs = set(required_fields) - set(kwargs)
         if missing_kwargs:
             msg = "Missing required field(s) for {type}: ({fields})."
@@ -50,34 +94,12 @@ class _STIXBase(collections.Mapping):
             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 (prop_name in kwargs and
-                        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.get('default', lambda x: ''))(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)
+            if isinstance(prop_metadata, dict):
+                self._handle_old_style_property(prop_name, prop_metadata, kwargs)
+            else:  # This is a Property Subclasses
+                # self.check_property(prop_name, prop_metadata, kwargs)
+                pass
 
         self._inner = kwargs
 
@@ -92,10 +114,13 @@ class _STIXBase(collections.Mapping):
 
     # Handle attribute access just like key access
     def __getattr__(self, name):
+        if name.startswith('_'):
+            return super(_STIXBase, self).__getattr__(name)
         return self.get(name)
 
     def __setattr__(self, name, value):
-        if name != '_inner':
+        if name != '_inner' and not name.startswith("_STIXBase__"):
+            print(name)
             raise ValueError("Cannot modify properties after creation.")
         super(_STIXBase, self).__setattr__(name, value)
 
diff --git a/stix2/properties.py b/stix2/properties.py
new file mode 100644
index 0000000..8ca92d0
--- /dev/null
+++ b/stix2/properties.py
@@ -0,0 +1,110 @@
+import uuid
+
+
+class Property(object):
+    """Represent a property of STIX data type.
+
+    Subclasses can define the following attributes as keyword arguments to
+    __init__():
+
+    - `required` - If `True`, the property must be provided when creating an
+        object with that property. No default value exists for these properties.
+        (Default: `False`)
+    - `fixed` - This provides a constant default value. Users are free to
+        provide this value explicity when constructing an object (which allows
+        you to copy *all* values from an existing object to a new object), but
+        if the user provides a value other than the `fixed` value, it will raise
+        an error. This is semantically equivalent to defining both:
+        - a `validate()` function that checks if the value matches the fixed
+          value, and
+        - a `default()` function that returns the fixed value.
+        (Default: `None`)
+
+    Subclasses can also define the following functions.
+
+    - `def clean(self, value) -> any:`
+        - Transform `value` into a valid value for this property. This should
+          raise a ValueError if such no such transformation is possible.
+    - `def validate(self, value) -> any:`
+        - check that `value` is valid for this property. This should return
+          a valid value (possibly modified) for this property, or raise a
+          ValueError if the value is not valid.
+          (Default: if `clean` is defined, it will attempt to call `clean` and
+          return the result or pass on a ValueError that `clean` raises. If
+          `clean` is not defined, this will return `value` unmodified).
+    - `def default(self):`
+        - provide a default value for this property.
+        - `default()` can return the special value `NOW` to use the current
+            time. This is useful when several timestamps in the same object need
+            to use the same default value, so calling now() for each field--
+            likely several microseconds apart-- does not work.
+
+    Subclasses can instead provide lambda functions for `clean`, and `default`
+    as keyword arguments. `validate` should not be provided as a lambda since
+    lambdas cannot raise their own exceptions.
+    """
+
+    def __init__(self, required=False, fixed=None, clean=None, validate=None, default=None):
+        self.required = required
+        if fixed:
+            self.validate = lambda x: x == fixed
+            self.default = lambda: fixed
+        if clean:
+            self.clean = clean
+        if validate:
+            self.validate = validate
+        if default:
+            self.default = default
+
+    def clean(self, value):
+        raise NotImplementedError
+
+    def validate(self, value):
+        try:
+            value = self.clean(value)
+        except NotImplemetedError:
+            pass
+        return value
+
+
+class TypeProperty(Property):
+
+    def __init__(self, type):
+        self.type = type
+
+    def validate(self, value):
+        if value != self.type:
+            raise ValueError("Invalid type value")
+
+    def default(self):
+        return self.type
+
+
+class List(Property):
+
+    def __init__(self, contained):
+        """
+        contained should be a type whose constructor creates an object from the value
+        """
+        self.contained = contained
+
+    def validate(self, value):
+        # TODO: ensure iterable
+        for item in value:
+            self.contained.validate(item)
+
+    def clean(self, value):
+        return [self.contained(x) for x in value]
+
+
+class IDProperty(Property):
+
+    def __init__(self, type):
+        self.type = type
+
+    def validate(self, value):
+        # TODO: validate GUID as well
+        return value.startswith(self.type + "--")
+
+    def default(self):
+        return self.type + "--" + str(uuid.uuid4())
diff --git a/stix2/test/test_properties.py b/stix2/test/test_properties.py
new file mode 100644
index 0000000..85adc34
--- /dev/null
+++ b/stix2/test/test_properties.py
@@ -0,0 +1,54 @@
+import pytest
+
+from stix2.properties import Property, IDProperty
+
+
+def test_property():
+    p = Property()
+
+    assert p.required is False
+
+
+def test_basic_validate():
+    class Prop(Property):
+
+        def validate(self, value):
+            if value == 42:
+                return value
+            else:
+                raise ValueError("Must be 42")
+
+    p = Prop()
+
+    assert p.validate(42) == 42
+    with pytest.raises(ValueError):
+        p.validate(41)
+
+
+def test_default_field():
+    class Prop(Property):
+
+        def default(self):
+            return 77
+
+    p = Prop()
+
+    assert p.default() == 77
+
+
+def test_fixed_property():
+    p = Property(fixed="2.0")
+
+    assert p.validate("2.0") is True
+    assert p.validate("x") is False
+    assert p.validate(2.0) is False
+
+    assert p.default() == "2.0"
+
+
+def test_id_property():
+    idprop = IDProperty('my-type')
+
+    assert idprop.validate('my-type--90aaca8a-1110-5d32-956d-ac2f34a1bd8c') is True
+    assert idprop.validate('not-my-type--90aaca8a-1110-5d32-956d-ac2f34a1bd8c') is False
+    assert idprop.validate(idprop.default())