diff --git a/EssentialCSharp.Web.Tests/ReferralServiceTests.cs b/EssentialCSharp.Web.Tests/ReferralServiceTests.cs
new file mode 100644
index 00000000..2f99f677
--- /dev/null
+++ b/EssentialCSharp.Web.Tests/ReferralServiceTests.cs
@@ -0,0 +1,119 @@
+using System.Security.Claims;
+using EssentialCSharp.Web.Areas.Identity.Data;
+using EssentialCSharp.Web.Data;
+using EssentialCSharp.Web.Extensions;
+using EssentialCSharp.Web.Services.Referrals;
+using Microsoft.AspNetCore.Identity;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace EssentialCSharp.Web.Tests;
+
+public class ReferralServiceTests : IntegrationTestBase
+{
+    [Test]
+    public async Task EnsureReferralIdAsync_NullUser_ReturnsNull()
+    {
+        string? referralId = await InServiceScopeAsync(async services =>
+            await services.GetRequiredService<IReferralService>().EnsureReferralIdAsync(null));
+
+        await Assert.That(referralId).IsNull();
+    }
+
+    [Test]
+    public async Task TrackReferralAsync_ExistingReferrer_IncrementsReferralCount()
+    {
+        string userId = CreateUserId();
+        string referralId = await CreateUserWithReferralIdAsync(userId);
+        string unrelatedUserId = CreateUserId();
+        await CreateUserAsync(unrelatedUserId);
+
+        InServiceScope(services =>
+            services.GetRequiredService<IReferralService>().TrackReferralAsync(referralId, null));
+
+        int referralCount = await GetReferralCountAsync(userId);
+        await Assert.That(referralCount).IsEqualTo(1);
+        int unrelatedReferralCount = await GetReferralCountAsync(unrelatedUserId);
+        await Assert.That(unrelatedReferralCount).IsEqualTo(0);
+    }
+
+    [Test]
+    public async Task TrackReferralAsync_UserHasSameReferralId_DoesNotIncrementReferralCount()
+    {
+        string userId = CreateUserId();
+        string referralId = await CreateUserWithReferralIdAsync(userId);
+        ClaimsPrincipal user = new(new ClaimsIdentity(
+        [
+            new Claim(ClaimsExtensions.ReferrerIdClaimType, referralId)
+        ]));
+
+        InServiceScope(services =>
+            services.GetRequiredService<IReferralService>().TrackReferralAsync(referralId, user));
+
+        int referralCount = await GetReferralCountAsync(userId);
+        await Assert.That(referralCount).IsEqualTo(0);
+    }
+
+    [Test]
+    public async Task TrackReferralAsync_UnknownReferralId_DoesNotThrowOrChangeUser()
+    {
+        string userId = CreateUserId();
+        await CreateUserAsync(userId);
+
+        InServiceScope(services =>
+            services.GetRequiredService<IReferralService>().TrackReferralAsync("unknown", null));
+
+        int referralCount = await GetReferralCountAsync(userId);
+        await Assert.That(referralCount).IsEqualTo(0);
+    }
+
+    private async Task<string> CreateUserWithReferralIdAsync(string userId)
+    {
+        await CreateUserAsync(userId);
+
+        return await InServiceScopeAsync(async services =>
+        {
+            UserManager<EssentialCSharpWebUser> userManager =
+                services.GetRequiredService<UserManager<EssentialCSharpWebUser>>();
+            EssentialCSharpWebUser user = await userManager.FindByIdAsync(userId)
+                ?? throw new InvalidOperationException($"Test user '{userId}' was not created.");
+
+            string referralId = await services.GetRequiredService<IReferralService>()
+                .EnsureReferralIdAsync(user)
+                ?? throw new InvalidOperationException("A referral ID was not generated.");
+            return referralId;
+        });
+    }
+
+    private async Task CreateUserAsync(string userId)
+    {
+        await InServiceScopeAsync(async services =>
+        {
+            UserManager<EssentialCSharpWebUser> userManager =
+                services.GetRequiredService<UserManager<EssentialCSharpWebUser>>();
+            IdentityResult result = await userManager.CreateAsync(
+                new EssentialCSharpWebUser
+                {
+                    Id = userId,
+                    UserName = $"{userId}@example.test",
+                    Email = $"{userId}@example.test"
+                });
+            if (!result.Succeeded)
+            {
+                throw new InvalidOperationException(
+                    $"Could not create test user: {string.Join(", ", result.Errors.Select(error => error.Description))}");
+            }
+        });
+    }
+
+    private async Task<int> GetReferralCountAsync(string userId)
+    {
+        return await InServiceScopeAsync(async services =>
+            await services.GetRequiredService<EssentialCSharpWebContext>().Users
+                .Where(user => user.Id == userId)
+                .Select(user => user.ReferralCount)
+                .SingleAsync());
+    }
+
+    private static string CreateUserId() => $"user-{Guid.NewGuid():N}";
+}