diff --git a/mongomap/base.py b/mongomap/base.py
index a906fbc6..c2bfa0fe 100644
--- a/mongomap/base.py
+++ b/mongomap/base.py
@@ -1,5 +1,7 @@
 from collection import CollectionManager
 
+import pymongo
+
 
 class ValidationError(Exception):
     pass
@@ -65,6 +67,25 @@ class BaseField(object):
         pass
 
 
+class ObjectIdField(BaseField):
+    """An field wrapper around MongoDB's ObjectIds.
+    """
+    
+    def _to_python(self, value):
+        return str(value)
+
+    def _to_mongo(self, value):
+        if not isinstance(value, pymongo.objectid.ObjectId):
+            return pymongo.objectid.ObjectId(value)
+        return value
+
+    def _validate(self, value):
+        try:
+            pymongo.objectid.ObjectId(str(value))
+        except:
+            raise ValidationError('Invalid Object ID')
+
+
 class DocumentMetaclass(type):
     """Metaclass for all documents.
     """
@@ -76,7 +97,6 @@ class DocumentMetaclass(type):
             return super_new(cls, name, bases, attrs)
 
         doc_fields = {}
-
         # Include all fields present in superclasses
         for base in bases:
             if hasattr(base, '_fields'):
@@ -85,7 +105,6 @@ class DocumentMetaclass(type):
         # Add the document's fields to the _fields attribute
         for attr_name, attr_value in attrs.items():
             if issubclass(attr_value.__class__, BaseField):
-                #print attr_value.name
                 if not attr_value.name:
                     attr_value.name = attr_name
                 doc_fields[attr_name] = attr_value
@@ -114,22 +133,15 @@ class TopLevelDocumentMetaclass(DocumentMetaclass):
         for base in bases:
             if hasattr(base, '_meta') and 'collection' in base._meta:
                 collection = base._meta['collection']
-        
-        # Get primary key field
-        object_id_field = None
-        for attr_name, attr_value in attrs.items():
-            if issubclass(attr_value.__class__, BaseField):
-                if hasattr(attr_value, 'object_id') and attr_value.object_id:
-                    object_id_field = attr_name
-                    attr_value.required = True
 
         meta = {
             'collection': collection,
-            'object_id_field': object_id_field,
         }
         meta.update(attrs.get('meta', {}))
         attrs['_meta'] = meta
 
+        attrs['_id'] = ObjectIdField()
+
         # Set up collection manager, needs the class to have fields so use
         # DocumentMetaclass before instantiating CollectionManager object
         new_class = super_new(cls, name, bases, attrs)
diff --git a/mongomap/collection.py b/mongomap/collection.py
index 5160fd8a..5016a1a5 100644
--- a/mongomap/collection.py
+++ b/mongomap/collection.py
@@ -10,7 +10,6 @@ class CollectionManager(object):
         self._collection_name = document._meta['collection']
         # This will create the collection if it doesn't exist
         self._collection = db[self._collection_name]
-        self._id_field = document._meta['object_id_field']
 
     def _save_document(self, document):
         """Save the provided document to the collection.
diff --git a/mongomap/fields.py b/mongomap/fields.py
index 61bfb357..3003e3c9 100644
--- a/mongomap/fields.py
+++ b/mongomap/fields.py
@@ -1,20 +1,20 @@
-from base import BaseField, ValidationError
+from base import BaseField, ObjectIdField, ValidationError
 from document import EmbeddedDocument
  
 import re
+import pymongo
 
 
 __all__ = ['StringField', 'IntField', 'EmbeddedDocumentField', 
-           'ValidationError']
+           'ObjectIdField', 'ValidationError']
 
 
 class StringField(BaseField):
     """A unicode string field.
     """
     
-    def __init__(self, regex=None, max_length=None, object_id=False, **kwargs):
+    def __init__(self, regex=None, max_length=None, **kwargs):
         self.regex = re.compile(regex) if regex else None
-        self.object_id = object_id
         self.max_length = max_length
         super(StringField, self).__init__(**kwargs)
 
diff --git a/tests/document.py b/tests/document.py
index 57ed4dbb..7a6e1c4f 100644
--- a/tests/document.py
+++ b/tests/document.py
@@ -31,6 +31,7 @@ class DocumentTest(unittest.TestCase):
         self.assertEqual(Person._fields['name'], name_field)
         self.assertEqual(Person._fields['age'], age_field)
         self.assertFalse('non_field' in Person._fields)
+        self.assertTrue('_id' in Person._fields)
         # Test iteration over fields
         fields = list(Person())
         self.assertTrue('name' in fields and 'age' in fields)
@@ -67,7 +68,8 @@ class DocumentTest(unittest.TestCase):
         person['name'] = 'Another User'
         self.assertEquals(person['name'], 'Another User')
 
-        self.assertEquals(len(person), 2)
+        # Length = length(assigned fields + _id)
+        self.assertEquals(len(person), 3)
 
         self.assertTrue('age' in person)
         person.age = None
@@ -81,6 +83,7 @@ class DocumentTest(unittest.TestCase):
             content = StringField()
         
         self.assertTrue('content' in Comment._fields)
+        self.assertFalse('_id' in Comment._fields)
         self.assertFalse(hasattr(Comment, '_meta'))
 
     def test_save(self):
@@ -95,6 +98,18 @@ class DocumentTest(unittest.TestCase):
         self.assertEqual(person_obj['name'], 'Test User')
         self.assertEqual(person_obj['age'], 30)
 
+    def test_save_custom_id(self):
+        """Ensure that a document may be saved with a custom _id.
+        """
+        # Create person object and save it to the database
+        person = self.Person(name='Test User', age=30, 
+                             _id='497ce96f395f2f052a494fd4')
+        person.save()
+        # Ensure that the object is in the database with the correct _id
+        collection = self.db[self.Person._meta['collection']]
+        person_obj = collection.find_one({'name': 'Test User'})
+        self.assertEqual(str(person_obj['_id']), '497ce96f395f2f052a494fd4')
+
     def test_save_embedded_document(self):
         """Ensure that a document with an embedded document field may be 
         saved in the database.
diff --git a/tests/fields.py b/tests/fields.py
index f657debb..c4af2be0 100644
--- a/tests/fields.py
+++ b/tests/fields.py
@@ -24,7 +24,7 @@ class FieldTest(unittest.TestCase):
         """Ensure that required field constraints are enforced.
         """
         class Person(Document):
-            name = StringField(object_id=True)
+            name = StringField(required=True)
             age = IntField(required=True)
             userid = StringField()
 
@@ -36,6 +36,18 @@ class FieldTest(unittest.TestCase):
         self.assertRaises(ValidationError, person.__setattr__, 'age', None)
         person.userid = None
 
+    def test_object_id_validation(self):
+        """Ensure that invalid values cannot be assigned to string fields.
+        """
+        class Person(Document):
+            name = StringField()
+        
+        person = Person(name='Test User')
+        self.assertRaises(AttributeError, getattr, person, '_id')
+        self.assertRaises(ValidationError, person.__setattr__, '_id', 47)
+        self.assertRaises(ValidationError, person.__setattr__, '_id', 'abc')
+        person._id = '497ce96f395f2f052a494fd4'
+
     def test_string_validation(self):
         """Ensure that invalid values cannot be assigned to string fields.
         """