diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4c2969f..80d6d4e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,6 +21,7 @@ jobs:
       - name: Run Tests
         env:
           CB_PROJECT_ID: ${{ secrets.CB_PROJECT_ID }}
+          CB_API_KEY: ${{ secrets.CB_API_KEY }}
         run: dotnet test
 
   release:
diff --git a/Commonbase.Examples/ChatCompletion.cs b/Commonbase.Examples/ChatCompletion.cs
index e69dc43..bf5fc35 100644
--- a/Commonbase.Examples/ChatCompletion.cs
+++ b/Commonbase.Examples/ChatCompletion.cs
@@ -4,7 +4,7 @@ public static class ChatCompletionExample
 {
   public static async Task RunAsync()
   {
-    CommonbaseClient client = new();
+    CommonbaseClient client = new(apiKey: Program.CB_API_KEY!);
 
     string systemMessage = "You help people with geography.";
     ChatMessage[] messages = new[] {
diff --git a/Commonbase.Examples/Program.cs b/Commonbase.Examples/Program.cs
index 08c20c4..70fb70f 100644
--- a/Commonbase.Examples/Program.cs
+++ b/Commonbase.Examples/Program.cs
@@ -5,12 +5,15 @@ public class Program
   // Set a CB_PROJECT_ID environment variable to run these examples without writing any code.
   // You can also hard-code your project ID here if you prefer.
   public static string? CB_PROJECT_ID = Environment.GetEnvironmentVariable("CB_PROJECT_ID");
+  public static string? CB_API_KEY = Environment.GetEnvironmentVariable("CB_API_KEY");
 
   public static async Task Main()
   {
-    if (string.IsNullOrWhiteSpace(CB_PROJECT_ID))
+    if (string.IsNullOrWhiteSpace(CB_PROJECT_ID) || string.IsNullOrWhiteSpace(CB_API_KEY))
     {
-      Console.Error.WriteLine("Please set the CB_PROJECT_ID environment variable to your Commonbase Project ID.");
+      Console.Error.WriteLine(
+        "Please set the CB_PROJECT_ID and CB_API_KEY environment variables to run these examples."
+      );
       return;
     }
 
diff --git a/Commonbase.Examples/StreamingCompletion.cs b/Commonbase.Examples/StreamingCompletion.cs
index 1bef01f..12e7acb 100644
--- a/Commonbase.Examples/StreamingCompletion.cs
+++ b/Commonbase.Examples/StreamingCompletion.cs
@@ -4,7 +4,8 @@ public static class StreamingCompletionExample
 {
   public static async Task RunAsync()
   {
-    CommonbaseClient client = new();
+    CommonbaseClient client = new(apiKey: Program.CB_API_KEY!);
+
     string prompt = "Write me an essay about artificial intelligence.";
 
     var stream = client.StreamCompletionAsync(
diff --git a/Commonbase.Examples/TextCompletion.cs b/Commonbase.Examples/TextCompletion.cs
index 2612615..7ff8406 100644
--- a/Commonbase.Examples/TextCompletion.cs
+++ b/Commonbase.Examples/TextCompletion.cs
@@ -4,7 +4,8 @@ public static class TextCompletionExample
 {
   public static async Task RunAsync()
   {
-    CommonbaseClient client = new();
+    CommonbaseClient client = new(apiKey: Program.CB_API_KEY!);
+
     string prompt = "Hello, what is your name?";
 
     Console.WriteLine("\n=======================================================");
diff --git a/Commonbase.Tests/ClientTests.cs b/Commonbase.Tests/ClientTests.cs
index e210035..c2c592b 100644
--- a/Commonbase.Tests/ClientTests.cs
+++ b/Commonbase.Tests/ClientTests.cs
@@ -8,10 +8,19 @@ public ClientTests()
   {
     Client = new CommonbaseClient(new ClientOptions
     {
-      ProjectId = Environment.GetEnvironmentVariable("CB_PROJECT_ID")
+      ApiKey = Environment.GetEnvironmentVariable("CB_API_KEY")!,
+      ProjectId = Environment.GetEnvironmentVariable("CB_PROJECT_ID")!
     });
   }
 
+  [Fact]
+  public void ShouldThrowArgumentExceptionOnNoApiKey()
+  {
+    Assert.Throws<ArgumentException>(
+      () => new CommonbaseClient(apiKey: "")
+    );
+  }
+
   [Fact]
   public async void ShouldFailOnInvalidProjectId()
   {
diff --git a/Commonbase/CommonbaseClient.cs b/Commonbase/CommonbaseClient.cs
index 7674312..d19bfe1 100644
--- a/Commonbase/CommonbaseClient.cs
+++ b/Commonbase/CommonbaseClient.cs
@@ -4,7 +4,11 @@
 
 namespace Commonbase;
 
-public record ClientOptions(string? ProjectId = null, string? ApiKey = null);
+public record ClientOptions
+{
+  public required string ApiKey { get; init; }
+  public string? ProjectId { get; init; }
+}
 
 public class CommonbaseClient
 {
@@ -21,9 +25,15 @@ static CommonbaseClient()
   private HttpClient HttpClient;
 
   private ClientOptions clientOptions;
-  public CommonbaseClient(ClientOptions? options = null)
+  public CommonbaseClient(string apiKey) : this(new ClientOptions { ApiKey = apiKey }) { }
+  public CommonbaseClient(ClientOptions options)
   {
-    clientOptions = options ?? new ClientOptions();
+    if (string.IsNullOrWhiteSpace(options.ApiKey))
+    {
+      throw new ArgumentException("Api Key must not be null or empty.", nameof(options.ApiKey));
+    }
+
+    clientOptions = options;
     HttpClient = new HttpClient();
   }
 
@@ -59,6 +69,7 @@ private async Task<HttpResponseMessage> SendCompletionRequestAsync(
 
     request.Content = body;
 
+    request.Headers.Add("Authorization", clientOptions.ApiKey);
     request.Headers.Add("User-Agent", $"commonbase-dotnet/{ClientVersion}");
 
     if (stream)
diff --git a/README.md b/README.md
index 9e3a8d0..7daef4a 100644
--- a/README.md
+++ b/README.md
@@ -16,24 +16,25 @@ dotnet package add Commonbase
 
 ## Usage
 
-A project ID is required for all Commonbase requests. You can find your project ID
-in the [Commonbase Dashboard](https://commonbase.com/).
+A Project ID and API Key are required for all Commonbase requests. You can find your Project ID
+and generate an API Key in the [Commonbase Dashboard](https://commonbase.com/).
 
-To create text and chat completions, use `CommonbaseClient.CreateCompletionAsync`:
+To create a completion, configure a `CommonbaseClient` with your API Key and provide your Project
+ID and prompt to `CreateCompletionAsync`:
 
 ```c#
 using Commonbase;
 
-CommonbaseClient client = new();
+CommonbaseClient client = new(apiKey: "API_KEY");
 
 var response = await client.CreateCompletionAsync(
   prompt: "Hello!",
-  projectId: "<your_project_id>"
+  projectId: "PROJECT_ID"
 );
 
 Console.WriteLine(response.BestResult);
 ```
 
-To stream a completion as it is generated, use `CommonbaseClient.StreamCompletionAsync`.
+To stream a completion as it is generated, use `StreamCompletionAsync`.
 
 For more examples, see [/Commonbase.Examples](https://github.com/commonbaseapp/commonbase-dotnet/tree/main/Commonbase.Examples).