diff --git a/registrasion/controllers/cart.py b/registrasion/controllers/cart.py
index 06eef3ef..45796781 100644
--- a/registrasion/controllers/cart.py
+++ b/registrasion/controllers/cart.py
@@ -139,6 +139,7 @@ class CartController(object):
         # It's not valid for users to re-enter a voucher they already have
         user_carts_with_voucher = rego.Cart.objects.filter(
             user=self.cart.user,
+            released=False,
             vouchers=voucher,
         )
         if len(user_carts_with_voucher) > 0:
diff --git a/registrasion/controllers/conditions.py b/registrasion/controllers/conditions.py
index 914a2092..20320218 100644
--- a/registrasion/controllers/conditions.py
+++ b/registrasion/controllers/conditions.py
@@ -44,7 +44,7 @@ class CategoryConditionController(ConditionController):
         ''' returns True if the user has a product from a category that invokes
         this condition in one of their carts '''
 
-        carts = rego.Cart.objects.filter(user=user)
+        carts = rego.Cart.objects.filter(user=user, released=False)
         enabling_products = rego.Product.objects.filter(
             category=self.condition.enabling_category)
         products = rego.ProductItem.objects.filter(
@@ -64,7 +64,7 @@ class ProductConditionController(ConditionController):
         ''' returns True if the user has a product that invokes this
         condition in one of their carts '''
 
-        carts = rego.Cart.objects.filter(user=user)
+        carts = rego.Cart.objects.filter(user=user, released=False)
         products = rego.ProductItem.objects.filter(
             cart=carts,
             product=self.condition.enabling_products.all())
diff --git a/registrasion/controllers/discount.py b/registrasion/controllers/discount.py
index dcb83dfd..7d6a959a 100644
--- a/registrasion/controllers/discount.py
+++ b/registrasion/controllers/discount.py
@@ -12,6 +12,11 @@ class DiscountAndQuantity(object):
         self.clause = clause
         self.quantity = quantity
 
+    def __repr__(self):
+        print "(discount=%s, clause=%s, quantity=%d)" % (
+            self.discount, self.clause, self.quantity,
+        )
+
 
 def available_discounts(user, categories, products):
     ''' Returns all discounts available to this user for the given categories
@@ -57,6 +62,7 @@ def available_discounts(user, categories, products):
         past_uses = rego.DiscountItem.objects.filter(
             cart__user=user,
             cart__active=False,  # Only past carts count
+            cart__released=False,  # You can reuse refunded discounts
             discount=discount.discount,
         )
         agg = past_uses.aggregate(Sum("quantity"))
diff --git a/registrasion/controllers/invoice.py b/registrasion/controllers/invoice.py
index 19da7525..fdea324b 100644
--- a/registrasion/controllers/invoice.py
+++ b/registrasion/controllers/invoice.py
@@ -1,6 +1,7 @@
 from decimal import Decimal
 from django.core.exceptions import ObjectDoesNotExist
 from django.core.exceptions import ValidationError
+from django.db import transaction
 from django.db.models import Sum
 
 from registrasion import models as rego
@@ -115,12 +116,13 @@ class InvoiceController(object):
         self.invoice.void = True
         self.invoice.save()
 
+    @transaction.atomic
     def pay(self, reference, amount):
         ''' Pays the invoice by the given amount. If the payment
         equals the total on the invoice, finalise the invoice.
         (NB should be transactional.)
         '''
-        if self.invoice.cart is not None:
+        if self.invoice.cart:
             cart = CartController(self.invoice.cart)
             cart.validate_cart()  # Raises ValidationError if invalid
 
@@ -145,8 +147,36 @@ class InvoiceController(object):
         if total == self.invoice.value:
             self.invoice.paid = True
 
-            cart = self.invoice.cart
-            cart.active = False
-            cart.save()
+            if self.invoice.cart:
+                cart = self.invoice.cart
+                cart.active = False
+                cart.save()
 
             self.invoice.save()
+
+    @transaction.atomic
+    def refund(self, reference, amount):
+        ''' Refunds the invoice by the given amount. The invoice is
+        marked as unpaid, and the underlying cart is marked as released.
+        '''
+
+        if self.invoice.void:
+            raise ValidationError("Void invoices cannot be refunded")
+
+        ''' Adds a payment '''
+        payment = rego.Payment.objects.create(
+            invoice=self.invoice,
+            reference=reference,
+            amount=0 - amount,
+        )
+        payment.save()
+
+        self.invoice.paid = False
+        self.invoice.void = True
+
+        if self.invoice.cart:
+            cart = self.invoice.cart
+            cart.released = True
+            cart.save()
+
+        self.invoice.save()
diff --git a/registrasion/models.py b/registrasion/models.py
index 2421adf9..2e1f95d1 100644
--- a/registrasion/models.py
+++ b/registrasion/models.py
@@ -423,7 +423,7 @@ class Cart(models.Model):
                 Q(time_last_updated__gt=(
                     timezone.now()-F('reservation_duration')
                                         ))) |
-            Q(active=False)
+            (Q(active=False) & Q(released=False))
         )
 
 
diff --git a/registrasion/tests/test_cart.py b/registrasion/tests/test_cart.py
index 5989f6e2..03d31b54 100644
--- a/registrasion/tests/test_cart.py
+++ b/registrasion/tests/test_cart.py
@@ -52,7 +52,7 @@ class RegistrationCartTestCase(SetTimeMixin, TestCase):
         cls.products = []
         for i in xrange(4):
             prod = rego.Product.objects.create(
-                name="Product 1",
+                name="Product " + str(i + 1),
                 description="This is a test product.",
                 category=cls.categories[i / 2],  # 2 products per category
                 price=Decimal("10.00"),
diff --git a/registrasion/tests/test_ceilings.py b/registrasion/tests/test_ceilings.py
index 0bbd6e9e..c39df39a 100644
--- a/registrasion/tests/test_ceilings.py
+++ b/registrasion/tests/test_ceilings.py
@@ -132,3 +132,21 @@ class CeilingsTestCases(RegistrationCartTestCase):
         self.add_timedelta(self.RESERVATION + datetime.timedelta(seconds=1))
         with self.assertRaises(ValidationError):
             first_cart.validate_cart()
+
+    def test_items_released_from_ceiling_by_refund(self):
+        self.make_ceiling("Limit ceiling", limit=1)
+
+        first_cart = CartController.for_user(self.USER_1)
+        first_cart.add_to_cart(self.PROD_1, 1)
+
+        first_cart.cart.active = False
+        first_cart.cart.save()
+
+        second_cart = CartController.for_user(self.USER_2)
+        with self.assertRaises(ValidationError):
+            second_cart.add_to_cart(self.PROD_1, 1)
+
+        first_cart.cart.released = True
+        first_cart.cart.save()
+
+        second_cart.add_to_cart(self.PROD_1, 1)
diff --git a/registrasion/tests/test_discount.py b/registrasion/tests/test_discount.py
index 222afc09..5f536e9b 100644
--- a/registrasion/tests/test_discount.py
+++ b/registrasion/tests/test_discount.py
@@ -377,3 +377,27 @@ class DiscountTestCase(RegistrationCartTestCase):
             [self.PROD_3, self.PROD_4],
         )
         self.assertEqual(2, len(discounts))
+
+    def test_discounts_are_released_by_refunds(self):
+        self.add_discount_prod_1_includes_prod_2(quantity=2)
+        cart = CartController.for_user(self.USER_1)
+        cart.add_to_cart(self.PROD_1, 1)  # Enable the discount
+        discounts = discount.available_discounts(self.USER_1, [], [self.PROD_2])
+        self.assertEqual(1, len(discounts))
+
+        cart.cart.active = False  # Keep discount enabled
+        cart.cart.save()
+
+        cart = CartController.for_user(self.USER_1)
+        cart.add_to_cart(self.PROD_2, 2) # The discount will be exhausted
+        cart.cart.active = False
+        cart.cart.save()
+
+        discounts = discount.available_discounts(self.USER_1, [], [self.PROD_2])
+        self.assertEqual(0, len(discounts))
+
+        cart.cart.released = True
+        cart.cart.save()
+
+        discounts = discount.available_discounts(self.USER_1, [], [self.PROD_2])
+        self.assertEqual(1, len(discounts))
diff --git a/registrasion/tests/test_enabling_condition.py b/registrasion/tests/test_enabling_condition.py
index e7155dea..2861e8db 100644
--- a/registrasion/tests/test_enabling_condition.py
+++ b/registrasion/tests/test_enabling_condition.py
@@ -233,3 +233,41 @@ class EnablingConditionTestCases(RegistrationCartTestCase):
 
         self.assertTrue(self.PROD_1 in prods)
         self.assertTrue(self.PROD_2 in prods)
+
+    def test_category_enabling_condition_fails_if_cart_refunded(self):
+        self.add_category_enabling_condition(mandatory=False)
+
+        cart = CartController.for_user(self.USER_1)
+        cart.add_to_cart(self.PROD_3, 1)
+
+        cart.cart.active = False
+        cart.cart.save()
+
+        cart_2 = CartController.for_user(self.USER_1)
+        cart_2.add_to_cart(self.PROD_1, 1)
+        cart_2.set_quantity(self.PROD_1, 0)
+
+        cart.cart.released = True
+        cart.cart.save()
+
+        with self.assertRaises(ValidationError):
+            cart_2.set_quantity(self.PROD_1, 1)
+
+    def test_product_enabling_condition_fails_if_cart_refunded(self):
+        self.add_product_enabling_condition(mandatory=False)
+
+        cart = CartController.for_user(self.USER_1)
+        cart.add_to_cart(self.PROD_2, 1)
+
+        cart.cart.active = False
+        cart.cart.save()
+
+        cart_2 = CartController.for_user(self.USER_1)
+        cart_2.add_to_cart(self.PROD_1, 1)
+        cart_2.set_quantity(self.PROD_1, 0)
+
+        cart.cart.released = True
+        cart.cart.save()
+
+        with self.assertRaises(ValidationError):
+            cart_2.set_quantity(self.PROD_1, 1)
diff --git a/registrasion/tests/test_refund.py b/registrasion/tests/test_refund.py
new file mode 100644
index 00000000..14811b78
--- /dev/null
+++ b/registrasion/tests/test_refund.py
@@ -0,0 +1,33 @@
+import datetime
+import pytz
+
+from decimal import Decimal
+from django.core.exceptions import ValidationError
+
+from registrasion import models as rego
+from registrasion.controllers.cart import CartController
+from registrasion.controllers.invoice import InvoiceController
+
+from test_cart import RegistrationCartTestCase
+
+UTC = pytz.timezone('UTC')
+
+
+class RefundTestCase(RegistrationCartTestCase):
+
+    def test_refund_marks_void_and_unpaid_and_cart_released(self):
+        current_cart = CartController.for_user(self.USER_1)
+
+        # Should be able to create an invoice after the product is added
+        current_cart.add_to_cart(self.PROD_1, 1)
+        invoice = InvoiceController.for_cart(current_cart.cart)
+
+        invoice.pay("A Payment!", invoice.invoice.value)
+        self.assertFalse(invoice.invoice.void)
+        self.assertTrue(invoice.invoice.paid)
+        self.assertFalse(invoice.invoice.cart.released)
+
+        invoice.refund("A Refund!", invoice.invoice.value)
+        self.assertTrue(invoice.invoice.void)
+        self.assertFalse(invoice.invoice.paid)
+        self.assertTrue(invoice.invoice.cart.released)
diff --git a/registrasion/tests/test_voucher.py b/registrasion/tests/test_voucher.py
index db59d094..abc7c8c3 100644
--- a/registrasion/tests/test_voucher.py
+++ b/registrasion/tests/test_voucher.py
@@ -126,3 +126,20 @@ class VoucherTestCases(RegistrationCartTestCase):
 
         with self.assertRaises(ValidationError):
             current_cart.apply_voucher(voucher.code)
+
+        return current_cart
+
+    def test_refund_releases_used_vouchers(self):
+        voucher = self.new_voucher(limit=2)
+        current_cart = CartController.for_user(self.USER_1)
+        current_cart.apply_voucher(voucher.code)
+
+        inv = InvoiceController.for_cart(current_cart.cart)
+        inv.pay("Hello!", inv.invoice.value)
+
+        current_cart = CartController.for_user(self.USER_1)
+        with self.assertRaises(ValidationError):
+            current_cart.apply_voucher(voucher.code)
+
+        inv.refund("Hello!", inv.invoice.value)
+        current_cart.apply_voucher(voucher.code)