diff --git a/MultiversalDiplomacy/Adjudicate/IPhaseAdjudicator.cs b/MultiversalDiplomacy/Adjudicate/IPhaseAdjudicator.cs
new file mode 100644
index 0000000..3bdfbf9
--- /dev/null
+++ b/MultiversalDiplomacy/Adjudicate/IPhaseAdjudicator.cs
@@ -0,0 +1,22 @@
+using MultiversalDiplomacy.Orders;
+
+namespace MultiversalDiplomacy.Adjudicate;
+
+/// <summary>
+/// An input handler for game phases.
+/// </summary>
+public interface IPhaseAdjudicator
+{
+    /// <summary>
+    /// Given a list of orders, determine which orders are valid for this adjudicator and
+    /// which should be rejected before adjudication. Adjudication should be performed on
+    /// all orders in the output for which <see cref="OrderValidation.Valid"/> is true.
+    /// </summary>
+    /// <param name="orders">Orders to validate for adjudication.</param>
+    /// <returns>
+    /// A list of order validation results. Note that this list may be longer than the input
+    /// list if illegal orders were replaced with hold orders, as there will be an invalid
+    /// result for the illegal order and a valid result for the replacement order.
+    /// </returns>
+    public IEnumerable<OrderValidation> ValidateOrders(IEnumerable<Order> orders);
+}
diff --git a/MultiversalDiplomacy/Adjudicate/OrderValidation.cs b/MultiversalDiplomacy/Adjudicate/OrderValidation.cs
new file mode 100644
index 0000000..553e47c
--- /dev/null
+++ b/MultiversalDiplomacy/Adjudicate/OrderValidation.cs
@@ -0,0 +1,46 @@
+using MultiversalDiplomacy.Orders;
+
+namespace MultiversalDiplomacy.Adjudicate;
+
+/// <summary>
+/// Represents the result of validating an order.
+/// </summary>
+public class OrderValidation
+{
+    /// <summary>
+    /// The order that was validated.
+    /// </summary>
+    public Order Order { get; }
+
+    /// <summary>
+    /// Whether the order is valid.
+    /// </summary>
+    public bool Valid { get; }
+
+    /// <summary>
+    /// The reason for the order validation result.
+    /// </summary>
+    public ValidationReason Reason { get; }
+
+    internal OrderValidation(Order order, bool valid, ValidationReason reason)
+    {
+        this.Order = order;
+        this.Valid = valid;
+        this.Reason = reason;
+    }
+}
+
+public static class OrderValidationExtensions
+{
+    /// <summary>
+    /// Create an <see cref="OrderValidation"/> accepting this order.
+    /// </summary>
+    public static OrderValidation Validate(this Order order, ValidationReason reason)
+        => new OrderValidation(order, true, reason);
+
+    /// <summary>
+    /// Create an <see cref="OrderValidation"/> rejecting this order.
+    /// </summary>
+    public static OrderValidation Invalidate(this Order order, ValidationReason reason)
+        => new OrderValidation(order, false, reason);
+}
diff --git a/MultiversalDiplomacy/Adjudicate/ValidationReason.cs b/MultiversalDiplomacy/Adjudicate/ValidationReason.cs
new file mode 100644
index 0000000..50b65d3
--- /dev/null
+++ b/MultiversalDiplomacy/Adjudicate/ValidationReason.cs
@@ -0,0 +1,24 @@
+namespace MultiversalDiplomacy.Adjudicate;
+
+public enum ValidationReason
+{
+    /// <summary>
+    /// The order is valid.
+    /// </summary>
+    Valid = 0,
+
+    /// <summary>
+    /// The order type is not valid for the current phase of the game.
+    /// </summary>
+    InvalidOrderTypeForPhase = 1,
+
+    /// <summary>
+    /// A hold order was created to replace an illegal order.
+    /// </summary>
+    IllegalOrderReplacedWithHold = 2,
+
+    /// <summary>
+    /// Another order was submitted that replaced this order.
+    /// </summary>
+    SupersededByLaterOrder = 3,
+}
\ No newline at end of file
diff --git a/MultiversalDiplomacy/Orders/Order.cs b/MultiversalDiplomacy/Orders/Order.cs
new file mode 100644
index 0000000..a8f1900
--- /dev/null
+++ b/MultiversalDiplomacy/Orders/Order.cs
@@ -0,0 +1,19 @@
+using MultiversalDiplomacy.Model;
+
+namespace MultiversalDiplomacy.Orders;
+
+/// <summary>
+/// A submitted action by a power.
+/// </summary>
+public abstract class Order
+{
+    /// <summary>
+    /// The power that submitted this order.
+    /// </summary>
+    public Power Power { get; }
+
+    public Order(Power power)
+    {
+        this.Power = power;
+    }
+}
diff --git a/MultiversalDiplomacyTests/AdjudicatorTests.cs b/MultiversalDiplomacyTests/AdjudicatorTests.cs
new file mode 100644
index 0000000..576c2d8
--- /dev/null
+++ b/MultiversalDiplomacyTests/AdjudicatorTests.cs
@@ -0,0 +1,28 @@
+using MultiversalDiplomacy.Adjudicate;
+using MultiversalDiplomacy.Model;
+using MultiversalDiplomacy.Orders;
+
+using NUnit.Framework;
+
+namespace MultiversalDiplomacyTests;
+
+public class AdjudicatorTests
+{
+    [Test]
+    public void OrderValidationTest()
+    {
+        IPhaseAdjudicator rubberStamp = new TestAdjudicator(orders => {
+            return orders.Select(o => o.Validate(ValidationReason.Valid));
+        });
+        Power power = new Power(nameof(Power));
+
+        Order order = new NullOrder(power);
+        List<Order> orders = new List<Order> { order };
+        IEnumerable<OrderValidation> results = rubberStamp.ValidateOrders(orders);
+
+        Assert.That(results.Count(), Is.EqualTo(1));
+        Assert.That(results.First().Order, Is.EqualTo(order));
+        Assert.That(results.First().Reason, Is.EqualTo(ValidationReason.Valid));
+        Assert.That(results.First().Valid, Is.True);
+    }
+}
\ No newline at end of file
diff --git a/MultiversalDiplomacyTests/NullOrder.cs b/MultiversalDiplomacyTests/NullOrder.cs
new file mode 100644
index 0000000..cab6c7a
--- /dev/null
+++ b/MultiversalDiplomacyTests/NullOrder.cs
@@ -0,0 +1,9 @@
+using MultiversalDiplomacy.Model;
+using MultiversalDiplomacy.Orders;
+
+namespace MultiversalDiplomacyTests;
+
+public class NullOrder : Order
+{
+    public NullOrder(Power power) : base(power) {}
+}
\ No newline at end of file
diff --git a/MultiversalDiplomacyTests/TestAdjudicator.cs b/MultiversalDiplomacyTests/TestAdjudicator.cs
new file mode 100644
index 0000000..0404b94
--- /dev/null
+++ b/MultiversalDiplomacyTests/TestAdjudicator.cs
@@ -0,0 +1,18 @@
+using MultiversalDiplomacy.Adjudicate;
+using MultiversalDiplomacy.Orders;
+
+namespace MultiversalDiplomacyTests;
+
+public class TestAdjudicator : IPhaseAdjudicator
+{
+    private Func<IEnumerable<Order>, IEnumerable<OrderValidation>> ValidateOrdersCallback;
+
+    public TestAdjudicator(
+        Func<IEnumerable<Order>, IEnumerable<OrderValidation>> validateOrdersCallback)
+    {
+        this.ValidateOrdersCallback = validateOrdersCallback;
+    }
+
+    public IEnumerable<OrderValidation> ValidateOrders(IEnumerable<Order> orders)
+        => this.ValidateOrdersCallback.Invoke(orders);
+}
\ No newline at end of file