diff --git a/src/allocation/domain/model.py b/src/allocation/domain/model.py
index 2eb22fb..20111be 100644
--- a/src/allocation/domain/model.py
+++ b/src/allocation/domain/model.py
@@ -4,6 +4,23 @@
 from typing import Optional, List
 
 
+class Product:
+    def __init__(self, sku: str, batches: List[Batch]):
+        self.sku = sku
+        self.batches = batches
+
+    def allocate(self, line: OrderLine) -> str:
+        try:
+            batch = next(
+                batch for batch in sorted(self.batches) if batch.can_allocate(line)
+            )
+            batch.allocate(line)
+            return batch.reference
+
+        except StopIteration:
+            raise OutOfStock(f"Out of stock for sku {line.sku}")
+
+
 @dataclass(eq=False)
 class OrderLine:
     order_id: str
diff --git a/tests/unit/test_product.py b/tests/unit/test_product.py
new file mode 100644
index 0000000..09b7a56
--- /dev/null
+++ b/tests/unit/test_product.py
@@ -0,0 +1,65 @@
+from datetime import date, timedelta
+from allocation.domain.model import Batch, OrderLine, OutOfStock, Product
+import pytest
+
+today = date.today()
+tomorrow = today + timedelta(days=1)
+later = today + timedelta(days=10)
+
+
+def test_prefers_current_stock_batches_to_shipments():
+    in_stock_batch = Batch("batch-001", "SMALL-TABLE", 100, eta=None)
+    shipment_batch = Batch("batch-002", "SMALL-TABLE", 100, eta=tomorrow)
+    product = Product(sku="SMALL-TABLE", batches=[shipment_batch, in_stock_batch])
+    order_line = OrderLine("order-123", "SMALL-TABLE", 10)
+
+    product.allocate(order_line)
+
+    assert in_stock_batch.available_quantity == 90
+    assert shipment_batch.available_quantity == 100
+
+
+def test_prefers_earlier_batches():
+    earliest_batch = Batch("batch-001", "SMALL-TABLE", 100, eta=today)
+    medium_batch = Batch("batch-002", "SMALL-TABLE", 100, eta=tomorrow)
+    latest_batch = Batch("batch-003", "SMALL-TABLE", 100, eta=later)
+    product = Product(sku="SMALL-TABLE", batches=[latest_batch, medium_batch, earliest_batch])
+    line = OrderLine("order-123", "SMALL-TABLE", 10)
+
+    product.allocate(line)
+
+    assert earliest_batch.available_quantity == 90
+    assert medium_batch.available_quantity == 100
+    assert latest_batch.available_quantity == 100
+
+
+def test_returns_allocated_batch_ref():
+    in_stock_batch = Batch("batch-001", "SMALL-TABLE", 100, eta=None)
+    shipment_batch = Batch("batch-002", "SMALL-TABLE", 100, eta=tomorrow)
+    product = Product(sku="SMALL-TABLE", batches=[shipment_batch, in_stock_batch])
+    order_line = OrderLine("order-123", "SMALL-TABLE", 10)
+
+    batch_ref = product.allocate(order_line)
+
+    assert batch_ref == in_stock_batch.reference
+
+
+def test_raises_out_of_stock_exception_if_cannot_allocate():
+    batch = Batch("batch-001", "SMALL-TABLE", 10, eta=today)
+    product = Product(sku="SMALL-TABLE", batches=[batch])
+
+    product.allocate(OrderLine("order-123", "SMALL-TABLE", 10))
+
+    with pytest.raises(OutOfStock, match="SMALL-TABLE"):
+        product.allocate(OrderLine("order-124", "SMALL-TABLE", 1))
+
+
+def test_incremental_available_quantity():
+    batch = Batch("batch-001", "SMALL-TABLE", 100, eta=None)
+    product = Product(sku="SMALL-TABLE", batches=[batch])
+
+    product.allocate(OrderLine("order-1", "SMALL-TABLE", 20))
+    assert batch.available_quantity == 80
+
+    product.allocate(OrderLine("order-2", "SMALL-TABLE", 30))
+    assert batch.available_quantity == 50