diff --git a/imagekit/models/fields/__init__.py b/imagekit/models/fields/__init__.py
index 34af0c7..2747ebf 100644
--- a/imagekit/models/fields/__init__.py
+++ b/imagekit/models/fields/__init__.py
@@ -8,34 +8,31 @@ from .utils import ImageSpecFileDescriptor, ImageKitMeta
 from ..receivers import configure_receivers
 from ...base import ImageSpec
 from ...utils import suggest_extension
+from ...specs import SpecHost
 
 
-class ImageSpecField(object):
+class ImageSpecField(SpecHost):
     """
     The heart and soul of the ImageKit library, ImageSpecField allows you to add
     variants of uploaded images to your models.
 
     """
     def __init__(self, processors=None, format=None, options=None,
-        image_field=None, storage=None, autoconvert=True,
-        image_cache_backend=None, image_cache_strategy=None):
+        image_field=None, storage=None, autoconvert=None,
+        image_cache_backend=None, image_cache_strategy=None, spec=None,
+        id=None):
 
         # The spec accepts a callable value for processors, but it
         # takes different arguments than the callable that ImageSpecField
         # expects, so we create a partial application and pass that instead.
         # TODO: Should we change the signatures to match? Even if `instance` is not part of the signature, it's accessible through the source file object's instance property.
-        p = lambda file: processors(instance=file.instance, file=file) if \
-                callable(processors) else processors
+        p = lambda file: processors(instance=file.instance,
+                file=file) if callable(processors) else processors
 
-        self.spec = ImageSpec(
-            processors=p,
-            format=format,
-            options=options,
-            storage=storage,
-            autoconvert=autoconvert,
-            image_cache_backend=image_cache_backend,
-            image_cache_strategy=image_cache_strategy,
-        )
+        SpecHost.__init__(self, processors=p, format=format,
+                options=options, storage=storage, autoconvert=autoconvert,
+                image_cache_backend=image_cache_backend,
+                image_cache_strategy=image_cache_strategy, spec=spec, id=id)
 
         self.image_field = image_field
 
@@ -59,6 +56,16 @@ class ImageSpecField(object):
             setattr(cls, '_ik', ik)
         ik.spec_fields.append(name)
 
+        # Generate a spec_id to register the spec with. The default spec id is
+        # "<app>:<model>_<field>"
+        if not self.spec_id:
+            self.spec_id = (u'%s:%s_%s' % (cls._meta.app_label,
+                    cls._meta.object_name, name)).lower()
+
+        # Register the spec with the id. This allows specs to be overridden
+        # later, from outside of the model definition.
+        self.register_spec(self.spec_id)
+
         # Register the field with the image_cache_backend
         try:
             self.spec.image_cache_backend.register_field(cls, self, name)
diff --git a/imagekit/specs.py b/imagekit/specs.py
index a81b82c..d2e0cba 100644
--- a/imagekit/specs.py
+++ b/imagekit/specs.py
@@ -164,3 +164,61 @@ class ImageSpec(BaseImageSpec):
                 storage.save(filename, content)
 
             return content
+
+
+class SpecHost(object):
+    """
+    An object that ostensibly has a spec attribute but really delegates to the
+    spec registry.
+
+    """
+    def __init__(self, processors=None, format=None, options=None,
+            storage=None, autoconvert=None, image_cache_backend=None,
+            image_cache_strategy=None, spec=None, id=None):
+
+        if spec:
+            if any([processors, format, options, storage, autoconvert,
+                    image_cache_backend, image_cache_strategy]):
+                raise TypeError('You can provide either an image spec or'
+                    ' arguments for the ImageSpec constructor, but not both.')
+        else:
+            spec = ImageSpec(
+                processors=processors,
+                format=format,
+                options=options,
+                storage=storage,
+                autoconvert=autoconvert,
+                image_cache_backend=image_cache_backend,
+                image_cache_strategy=image_cache_strategy,
+            )
+
+        self._original_spec = spec
+        self.spec_id = None
+
+        if id:
+            # If an id is given, register the spec immediately.
+            self.register_spec(id)
+
+    def register_spec(self, id):
+        """
+        Registers the spec with the specified id. Useful for when the id isn't
+        known when the instance is constructed (e.g. for ImageSpecFields whose
+        generated `spec_id`s are only known when they are contributed to a
+        class)
+
+        """
+        self.spec_id = id
+        spec_registry.register(id, self._original_spec)
+
+    @property
+    def spec(self):
+        """
+        Look up the spec by the spec id. We do this (instead of storing the
+        spec as an attribute) so that users can override apps' specs--without
+        having to edit model definitions--simply by registering another spec
+        with the same id.
+
+        """
+        if not getattr(self, 'spec_id', None):
+            raise Exception('Object %s has no spec id.' % self)
+        return spec_registry.get_spec(self.spec_id)