diff --git a/registrasion/controllers/invoice.py b/registrasion/controllers/invoice.py
new file mode 100644
index 0000000000000000000000000000000000000000..67a8c836646798b3356c2ca632873385fb885d3d
--- /dev/null
+++ b/registrasion/controllers/invoice.py
@@ -0,0 +1,137 @@
+from decimal import Decimal
+from django.core.exceptions import ObjectDoesNotExist
+from django.db.models import Avg, Min, Max, Sum
+
+from registrasion import models as rego
+
+from cart import CartController
+
+class InvoiceController(object):
+
+    def __init__(self, invoice):
+        self.invoice = invoice
+
+    @classmethod
+    def for_cart(cls, cart):
+        ''' Returns an invoice object for a given cart at its current revision.
+        If such an invoice does not exist, the cart is validated, and if valid,
+        an invoice is generated.'''
+
+        try:
+            invoice = rego.Invoice.objects.get(
+                cart=cart, cart_revision=cart.revision)
+        except ObjectDoesNotExist:
+            cart_controller = CartController(cart)
+            cart_controller.validate_cart() # Raises ValidationError on fail.
+            invoice = cls._generate(cart)
+
+        return InvoiceController(invoice)
+
+
+    @classmethod
+    def resolve_discount_value(cls, item):
+        try:
+            condition = rego.DiscountForProduct.objects.get(
+                discount=item.discount,
+                product=item.product
+            )
+        except ObjectDoesNotExist:
+            condition = rego.DiscountForCategory.objects.get(
+                discount=item.discount,
+                category=item.product.category
+            )
+        if condition.percentage is not None:
+            value = item.product.price * (condition.percentage / 100)
+        else:
+            value = condition.price
+        return value
+
+
+    @classmethod
+    def _generate(cls, cart):
+        ''' Generates an invoice for the given cart. '''
+        invoice = rego.Invoice.objects.create(
+            user=cart.user,
+            cart=cart,
+            cart_revision=cart.revision,
+            value=Decimal()
+        )
+        invoice.save()
+
+        # TODO: calculate line items.
+        product_items = rego.ProductItem.objects.filter(cart=cart)
+        discount_items = rego.DiscountItem.objects.filter(cart=cart)
+        invoice_value = Decimal()
+        for item in product_items:
+            line_item = rego.LineItem.objects.create(
+                invoice=invoice,
+                description=item.product.name,
+                quantity=item.quantity,
+                price=item.product.price,
+            )
+            line_item.save()
+            invoice_value += line_item.quantity * line_item.price
+
+        for item in discount_items:
+
+            line_item = rego.LineItem.objects.create(
+                invoice=invoice,
+                description=item.discount.description,
+                quantity=item.quantity,
+                price=cls.resolve_discount_value(item) * -1,
+            )
+            line_item.save()
+            invoice_value += line_item.quantity * line_item.price
+
+        # TODO: calculate line items from discounts
+        invoice.value = invoice_value
+        invoice.save()
+
+        return invoice
+
+
+    def is_valid(self):
+        ''' Returns true if the attached invoice is not void and it represents
+        a valid cart. '''
+        if self.invoice.void:
+            return False
+        if self.invoice.cart is not None:
+            if self.invoice.cart.revision != self.invoice.cart_revision:
+                return False
+        return True
+
+
+    def void(self):
+        ''' Voids the invoice. '''
+        self.invoice.void = True
+
+
+    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:
+            cart = CartController(self.invoice.cart)
+            cart.validate_cart() # Raises ValidationError if invalid
+
+        ''' Adds a payment '''
+        payment = rego.Payment.objects.create(
+            invoice=self.invoice,
+            reference=reference,
+            amount=amount,
+        )
+        payment.save()
+
+        payments = rego.Payment.objects .filter(invoice=self.invoice)
+        agg = payments.aggregate(Sum("amount"))
+        total = agg["amount__sum"]
+
+        if total==self.invoice.value:
+            self.invoice.paid = True
+
+            cart = self.invoice.cart
+            cart.active = False
+            cart.save()
+
+            self.invoice.save()