diff --git a/Tests/Singulink.Globalization.Currency.Tests/ImmutableSortedMoneySetTests/AddTests.cs b/Tests/Singulink.Globalization.Currency.Tests/ImmutableSortedMoneySetTests/AddTests.cs
new file mode 100644
index 0000000..bda62bd
--- /dev/null
+++ b/Tests/Singulink.Globalization.Currency.Tests/ImmutableSortedMoneySetTests/AddTests.cs
@@ -0,0 +1,100 @@
+using System.Collections.Generic;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Shouldly;
+
+namespace Singulink.Globalization.Tests.ImmutableSortedMoneySetTests;
+
+[TestClass]
+public class AddTests
+{
+    private static readonly Money Usd100 = new(100m, "USD");
+    private static readonly Money Cad50 = new(50m, "CAD");
+    private static readonly Money Eur25 = new(25m, "EUR");
+    private static readonly Currency DisallowedCurrency = new("Blah blah blah", "BBB", "$$", 2);
+    private static readonly ImmutableSortedMoneySet _immutableSet = [Usd100, Cad50];
+
+    // public void Add(Money value) tests
+
+    [TestMethod]
+    public void AddMoney_CurrencyExists_UpdatesValue()
+    {
+        var updatedSet = _immutableSet.Add(Usd100);
+
+        updatedSet.Count.ShouldBe(2);
+        updatedSet.ShouldBe(ImmutableSortedMoneySet.Create(Money.Create(200m, "USD"), Cad50));
+    }
+
+    [TestMethod]
+    public void AddMoney_NewCurrency_AddsValue()
+    {
+        var updatedSet = _immutableSet.Add(Eur25);
+
+        updatedSet.Count.ShouldBe(3);
+        updatedSet.ShouldBe(ImmutableSortedMoneySet.Create(Cad50, Eur25, Usd100));
+    }
+
+    [TestMethod]
+    public void AddMoney_DefaultValue_NoChange()
+    {
+        var updatedSet = _immutableSet.Add(default);
+
+        updatedSet.Count.ShouldBe(2);
+        updatedSet.ShouldBe(_immutableSet);
+    }
+
+    [TestMethod]
+    public void AddMoney_CurrencyDisallowed_ThrowsException()
+    {
+        var value = new Money(100, DisallowedCurrency);
+        Should.Throw<ArgumentException>(() => _immutableSet.Add(value));
+    }
+
+    // public void Add(decimal amount, string currencyCode) tests
+
+    [TestMethod]
+    public void AddByCurrencyCode_CurrencyExists_UpdatesValue()
+    {
+        var updatedSet = _immutableSet.Add(100, "USD");
+        updatedSet.Count.ShouldBe(2);
+        updatedSet.ShouldBe([Cad50, new(200m, "USD")]);
+    }
+
+    [TestMethod]
+    public void AddByCurrencyCode_NewCurrency_AddsValue()
+    {
+        var updatedSet = _immutableSet.Add(25m, "EUR");
+        updatedSet.Count.ShouldBe(3);
+        updatedSet.ShouldBe([Cad50, Eur25, Usd100]);
+    }
+
+    [TestMethod]
+    public void AddByCurrencyCode_CurrencyDisallowed_ThrowsArgumentException()
+    {
+        Should.Throw<ArgumentException>(() => _immutableSet.Add(100m, DisallowedCurrency.CurrencyCode));
+    }
+
+    // public void Add(decimal amount, Currency currency) tests
+
+    [TestMethod]
+    public void AddByCurrency_CurrencyExists_UpdatesValue()
+    {
+        var updatedSet = _immutableSet.Add(100m, Currency.Get("USD"));
+        updatedSet.Count.ShouldBe(2);
+        updatedSet.ShouldBe([Cad50, new(200m, "USD")]);
+    }
+
+    [TestMethod]
+    public void AddByCurrency_NewCurrency_AddsValue()
+    {
+        var updatedSet = _immutableSet.Add(25m, Currency.Get("EUR"));
+        updatedSet.Count.ShouldBe(3);
+        updatedSet.ShouldBe([Cad50, Eur25, Usd100]);
+    }
+
+    [TestMethod]
+    public void AddByCurrency_CurrencyDisallowed_ThrowsArgumentException()
+    {
+        _immutableSet.Count.ShouldBe(2);
+        Should.Throw<ArgumentException>(() => _immutableSet.Add(100m, DisallowedCurrency));
+    }
+}
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