diff --git a/registrasion/controllers/cart.py b/registrasion/controllers/cart.py
index 97e17594..f8cbd0f5 100644
--- a/registrasion/controllers/cart.py
+++ b/registrasion/controllers/cart.py
@@ -251,6 +251,37 @@ class CartController(object):
         if errors:
             raise(ValidationError(ve))
 
+    def _test_required_categories(self):
+        ''' Makes sure that the owner of this cart has satisfied all of the
+        required category constraints in the inventory (be it in this cart
+        or others). '''
+
+        required = set(inventory.Category.objects.filter(required=True))
+
+        items = commerce.ProductItem.objects.filter(
+            product__category__required=True,
+            cart__user=self.cart.user,
+        ).exclude(
+            cart__status=commerce.Cart.STATUS_RELEASED,
+        )
+
+        for item in items:
+            print item
+            required.remove(item.product.category)
+
+        errors = []
+        for category in required:
+            msg = "You must have at least one item from: %s" % category
+            errors.append((None, msg))
+
+        if errors:
+            raise ValidationError(errors)
+
+    def _append_errors(self, errors, ve):
+        for error in ve.error_list:
+            print error.message
+            errors.append(error.message[1])
+
     def validate_cart(self):
         ''' Determines whether the status of the current cart is valid;
         this is normally called before generating or paying an invoice '''
@@ -270,8 +301,12 @@ class CartController(object):
         try:
             self._test_limits(product_quantities)
         except ValidationError as ve:
-            for error in ve.error_list:
-                errors.append(error.message[1])
+            self._append_errors(errors, ve)
+
+        try:
+            self._test_required_categories()
+        except ValidationError as ve:
+            self._append_errors(errors, ve)
 
         # Validate the discounts
         discount_items = commerce.DiscountItem.objects.filter(cart=cart)
diff --git a/registrasion/tests/test_invoice.py b/registrasion/tests/test_invoice.py
index 1b687c0c..9e5b55cf 100644
--- a/registrasion/tests/test_invoice.py
+++ b/registrasion/tests/test_invoice.py
@@ -499,3 +499,37 @@ class InvoiceTestCase(RegistrationCartTestCase):
         invoice.pay("Paying into the void.", val, pre_validate=False)
         cn = self._credit_note_for_invoice(invoice.invoice)
         self.assertEqual(val, cn.credit_note.value)
+
+    def test_required_category_constraints_prevent_invoicing(self):
+        self.CAT_1.required = True
+        self.CAT_1.save()
+
+        cart = TestingCartController.for_user(self.USER_1)
+        cart.add_to_cart(self.PROD_3, 1)
+
+        # CAT_1 is required, we don't have CAT_1 yet
+        with self.assertRaises(ValidationError):
+            invoice = TestingInvoiceController.for_cart(cart.cart)
+
+        # Now that we have CAT_1, we can check out the cart
+        cart.add_to_cart(self.PROD_1, 1)
+        invoice = TestingInvoiceController.for_cart(cart.cart)
+
+        # Paying for the invoice should work fine
+        invoice.pay("Boop", invoice.invoice.value)
+
+        # We have an item in the first cart, so should be able to invoice
+        # for the second cart, even without CAT_1 in it.
+        cart = TestingCartController.for_user(self.USER_1)
+        cart.add_to_cart(self.PROD_3, 1)
+
+        invoice2 = TestingInvoiceController.for_cart(cart.cart)
+
+        # Void invoice2, and release the first cart
+        # now we don't have any CAT_1
+        invoice2.void()
+        invoice.refund()
+
+        # Now that we don't have CAT_1, we can't checkout this cart
+        with self.assertRaises(ValidationError):
+            invoice = TestingInvoiceController.for_cart(cart.cart)