diff --git a/nuget.config b/nuget.config
index 6338e895..a3246f9f 100644
--- a/nuget.config
+++ b/nuget.config
@@ -4,4 +4,12 @@
     <clear />
     <add key="Office" value="https://pkgs.dev.azure.com/office/_packaging/Office/nuget/v3/index.json"/>
   </packageSources>
+  <!-- Provide ADO_TOKEN for authenticated package restore:
+       ADO_TOKEN=$(azureauth ado token) dotnet restore -->
+  <packageSourceCredentials>
+    <Office>
+      <add key="Username" value="UnusedWithToken" />
+      <add key="ClearTextPassword" value="%ADO_TOKEN%" />
+    </Office>
+  </packageSourceCredentials>
 </configuration>
\ No newline at end of file
diff --git a/src/AzureAuth.Test/Commands/Ado/CommandTokenTest.cs b/src/AzureAuth.Test/Commands/Ado/CommandTokenTest.cs
index f13f8a8c..e92e0176 100644
--- a/src/AzureAuth.Test/Commands/Ado/CommandTokenTest.cs
+++ b/src/AzureAuth.Test/Commands/Ado/CommandTokenTest.cs
@@ -3,19 +3,68 @@
 
 namespace AzureAuth.Test.Commands.Ado
 {
+    using System;
+    using System.Collections.Generic;
+
     using FluentAssertions;
-    using Microsoft.Authentication.AzureAuth.Ado;
+    using AzureAuth.Test;
+    using Microsoft.Authentication.AzureAuth;
     using Microsoft.Authentication.AzureAuth.Commands.Ado;
+    using Microsoft.Authentication.MSALWrapper;
+    using Microsoft.Extensions.DependencyInjection;
+    using Microsoft.Extensions.Logging;
+    using Microsoft.IdentityModel.JsonWebTokens;
+    using Microsoft.Office.Lasso.Interfaces;
+    using Microsoft.Office.Lasso.Telemetry;
+    using Moq;
+    using NLog.Extensions.Logging;
+    using NLog.Targets;
     using NUnit.Framework;
 
     internal class CommandTokenTest
     {
+        private Mock<IEnv> mockEnv;
+        private Mock<ITelemetryService> mockTelemetry;
+        private Mock<IPublicClientAuth> mockPublicClientAuth;
+        private IServiceProvider serviceProvider;
+        private MemoryTarget logTarget;
+        private CommandExecuteEventData eventData;
+
+        [SetUp]
+        public void SetUp()
+        {
+            this.mockEnv = new Mock<IEnv>();
+            this.mockEnv.Setup(e => e.Get(It.IsAny<string>())).Returns((string)null);
+            this.mockTelemetry = new Mock<ITelemetryService>();
+            this.mockPublicClientAuth = new Mock<IPublicClientAuth>();
+            this.eventData = new CommandExecuteEventData();
+
+            // Setup in memory logging target with NLog - allows making assertions against what has been logged.
+            var loggingConfig = new NLog.Config.LoggingConfiguration();
+            this.logTarget = new MemoryTarget("memory_target")
+            {
+                Layout = "${message}" // Define a simple layout so we don't get timestamps in messages.
+            };
+            loggingConfig.AddTarget(this.logTarget);
+            loggingConfig.AddRuleForAllLevels(this.logTarget);
+
+            // Setup Dependency Injection container to provide logger.
+            this.serviceProvider = new ServiceCollection()
+                .AddLogging(loggingBuilder =>
+                {
+                    loggingBuilder.ClearProviders();
+                    loggingBuilder.SetMinimumLevel(LogLevel.Trace);
+                    loggingBuilder.AddNLog(loggingConfig);
+                })
+                .BuildServiceProvider();
+        }
+
         [TestCase("foobar", CommandToken.OutputMode.Token, "foobar")]
         [TestCase("foobar", CommandToken.OutputMode.HeaderValue, "Basic OmZvb2Jhcg==")]
         [TestCase("foobar", CommandToken.OutputMode.Header, "Authorization: Basic OmZvb2Jhcg==")]
         public void FormatToken_Basic(string input, CommandToken.OutputMode mode, string expected)
         {
-            CommandToken.FormatToken(input, mode, Authorization.Basic).Should().Be(expected);
+            CommandToken.FormatToken(input, mode, Microsoft.Authentication.AzureAuth.Ado.Authorization.Basic).Should().Be(expected);
         }
 
         [TestCase("foobar", CommandToken.OutputMode.Token, "foobar")]
@@ -23,7 +72,108 @@ public void FormatToken_Basic(string input, CommandToken.OutputMode mode, string
         [TestCase("foobar", CommandToken.OutputMode.Header, "Authorization: Bearer foobar")]
         public void FormatToken_Bearer(string input, CommandToken.OutputMode mode, string expected)
         {
-            CommandToken.FormatToken(input, mode, Authorization.Bearer).Should().Be(expected);
+            CommandToken.FormatToken(input, mode, Microsoft.Authentication.AzureAuth.Ado.Authorization.Bearer).Should().Be(expected);
+        }
+
+        [Test]
+        public void OnExecute_AzureAuthAdoPat_AlwaysUsed()
+        {
+            this.mockEnv.Setup(e => e.Get(EnvVars.AdoPat)).Returns("my-explicit-pat");
+
+            var command = new CommandToken();
+            var result = command.OnExecute(
+                this.serviceProvider.GetService<ILogger<CommandToken>>(),
+                this.mockEnv.Object,
+                this.mockTelemetry.Object,
+                this.mockPublicClientAuth.Object,
+                this.eventData);
+
+            result.Should().Be(0);
+            this.mockPublicClientAuth.Verify(
+                p => p.Token(It.IsAny<AuthParameters>(), It.IsAny<IEnumerable<AuthMode>>(), It.IsAny<string>(), It.IsAny<string>(), It.IsAny<TimeSpan>(), It.IsAny<EventData>()),
+                Times.Never);
+        }
+
+        [Test]
+        public void OnExecute_AdoPipeline_UsesSystemAccessToken()
+        {
+            this.mockEnv.Setup(e => e.Get(EnvVars.TfBuild)).Returns("True");
+            this.mockEnv.Setup(e => e.Get(EnvVars.SystemAccessToken)).Returns("pipeline-token");
+
+            var command = new CommandToken();
+            var result = command.OnExecute(
+                this.serviceProvider.GetService<ILogger<CommandToken>>(),
+                this.mockEnv.Object,
+                this.mockTelemetry.Object,
+                this.mockPublicClientAuth.Object,
+                this.eventData);
+
+            result.Should().Be(0);
+            this.mockPublicClientAuth.Verify(
+                p => p.Token(It.IsAny<AuthParameters>(), It.IsAny<IEnumerable<AuthMode>>(), It.IsAny<string>(), It.IsAny<string>(), It.IsAny<TimeSpan>(), It.IsAny<EventData>()),
+                Times.Never);
+        }
+
+        [Test]
+        public void OnExecute_AdoPipeline_NoSystemAccessToken_ReturnsError()
+        {
+            this.mockEnv.Setup(e => e.Get(EnvVars.TfBuild)).Returns("True");
+
+            var command = new CommandToken();
+            var result = command.OnExecute(
+                this.serviceProvider.GetService<ILogger<CommandToken>>(),
+                this.mockEnv.Object,
+                this.mockTelemetry.Object,
+                this.mockPublicClientAuth.Object,
+                this.eventData);
+
+            result.Should().Be(1);
+            this.logTarget.Logs.Should().Contain(l => l.Contains($"{EnvVars.SystemAccessToken} is not set"));
+        }
+
+        [Test]
+        public void OnExecute_NotAdoPipeline_SystemAccessTokenSet_WarnsAndContinues()
+        {
+            this.mockEnv.Setup(e => e.Get(EnvVars.SystemAccessToken)).Returns("stale-token");
+            var fakeTokenResult = new TokenResult(new JsonWebToken(Fake.Token), Guid.NewGuid());
+            this.mockPublicClientAuth
+                .Setup(p => p.Token(It.IsAny<AuthParameters>(), It.IsAny<IEnumerable<AuthMode>>(), It.IsAny<string>(), It.IsAny<string>(), It.IsAny<TimeSpan>(), It.IsAny<EventData>()))
+                .Returns(fakeTokenResult);
+
+            var command = new CommandToken();
+            var result = command.OnExecute(
+                this.serviceProvider.GetService<ILogger<CommandToken>>(),
+                this.mockEnv.Object,
+                this.mockTelemetry.Object,
+                this.mockPublicClientAuth.Object,
+                this.eventData);
+
+            result.Should().Be(0);
+            this.logTarget.Logs.Should().Contain(l => l.Contains("does not appear to be an Azure DevOps Pipeline environment"));
+
+            // Verify it fell through to AAD auth (ignored the SYSTEM_ACCESSTOKEN)
+            this.mockPublicClientAuth.Verify(
+                p => p.Token(It.IsAny<AuthParameters>(), It.IsAny<IEnumerable<AuthMode>>(), It.IsAny<string>(), It.IsAny<string>(), It.IsAny<TimeSpan>(), It.IsAny<EventData>()),
+                Times.Once);
+        }
+
+        [Test]
+        public void OnExecute_AdoPipeline_AzureAuthAdoPat_TakesPriority()
+        {
+            this.mockEnv.Setup(e => e.Get(EnvVars.AdoPat)).Returns("my-explicit-pat");
+            this.mockEnv.Setup(e => e.Get(EnvVars.TfBuild)).Returns("True");
+            this.mockEnv.Setup(e => e.Get(EnvVars.SystemAccessToken)).Returns("pipeline-token");
+
+            var command = new CommandToken();
+            var result = command.OnExecute(
+                this.serviceProvider.GetService<ILogger<CommandToken>>(),
+                this.mockEnv.Object,
+                this.mockTelemetry.Object,
+                this.mockPublicClientAuth.Object,
+                this.eventData);
+
+            result.Should().Be(0);
+            this.logTarget.Logs.Should().Contain(l => l.Contains(EnvVars.AdoPat));
         }
     }
 }
diff --git a/src/AzureAuth.Test/IEnvExtensionsTest.cs b/src/AzureAuth.Test/IEnvExtensionsTest.cs
index 3acb16ab..86ce993b 100644
--- a/src/AzureAuth.Test/IEnvExtensionsTest.cs
+++ b/src/AzureAuth.Test/IEnvExtensionsTest.cs
@@ -10,7 +10,6 @@ namespace AzureAuth.Test
     using Microsoft.Authentication.TestHelper;
     using Microsoft.Extensions.Logging;
     using Microsoft.Office.Lasso.Interfaces;
-    using Microsoft.Office.Lasso.Telemetry;
     using Moq;
     using NLog.Targets;
     using NUnit.Framework;
@@ -58,6 +57,20 @@ public void InteractiveAuth_IsEnabledIfEnvVarsAreNotSet()
             IEnvExtensions.InteractiveAuthDisabled(this.envMock.Object).Should().BeFalse();
         }
 
+        [TestCase("True", true)]
+        [TestCase("true", true)]
+        [TestCase("TRUE", true)]
+        [TestCase("False", false)]
+        [TestCase("", false)]
+        [TestCase(null, false)]
+        public void IsAdoPipeline_DetectsTfBuildEnvVar(string tfBuild, bool expected)
+        {
+            this.envMock.Setup(env => env.Get(It.IsAny<string>())).Returns((string)null);
+            this.envMock.Setup(e => e.Get(EnvVars.TfBuild)).Returns(tfBuild);
+
+            IEnvExtensions.IsAdoPipeline(this.envMock.Object).Should().Be(expected);
+        }
+
         [Test]
         public void ReadAuthModeFromEnvOrSetDefault_ReturnsDefault_WhenEnvVarIsEmpty()
         {
diff --git a/src/AzureAuth/Ado/PatFromEnv.cs b/src/AzureAuth/Ado/PatFromEnv.cs
index f02c2d48..4be4b78e 100644
--- a/src/AzureAuth/Ado/PatFromEnv.cs
+++ b/src/AzureAuth/Ado/PatFromEnv.cs
@@ -3,18 +3,12 @@
 
 namespace Microsoft.Authentication.AzureAuth.Ado
 {
-    using System;
     using System.Collections.Generic;
-    using System.Linq;
-    using System.Threading.Tasks;
 
-    using Microsoft.Authentication.MSALWrapper;
-    using Microsoft.Authentication.MSALWrapper.AuthFlow;
-    using Microsoft.Extensions.Logging;
     using Microsoft.Office.Lasso.Interfaces;
 
     /// <summary>
-    /// A class for getting an ADO PAT from an <see cref="IEnv"/> or an AAD access token through MSAL.
+    /// A class for getting an ADO PAT from an <see cref="IEnv"/>.
     /// </summary>
     public static class PatFromEnv
     {
diff --git a/src/AzureAuth/Commands/Ado/CommandToken.cs b/src/AzureAuth/Commands/Ado/CommandToken.cs
index 463bad73..2244cb00 100644
--- a/src/AzureAuth/Commands/Ado/CommandToken.cs
+++ b/src/AzureAuth/Commands/Ado/CommandToken.cs
@@ -89,13 +89,32 @@ public enum OutputMode
         /// <returns>An integer status code. 0 for success and non-zero for failure.</returns>
         public int OnExecute(ILogger<CommandToken> logger, IEnv env, ITelemetryService telemetryService, IPublicClientAuth publicClientAuth, CommandExecuteEventData eventData)
         {
-            // First attempt using a PAT.
+            // First attempt using a PAT from the environment.
             var pat = PatFromEnv.Get(env);
             if (pat.Exists)
             {
-                logger.LogDebug($"Using PAT from env var {pat.EnvVarSource}");
-                logger.LogInformation(FormatToken(pat.Value, this.Output, Authorization.Basic));
-                return 0;
+                // SYSTEM_ACCESSTOKEN should only be used inside an ADO Pipeline.
+                if (pat.EnvVarSource == EnvVars.SystemAccessToken && !env.IsAdoPipeline())
+                {
+                    logger.LogWarning(
+                        $"{EnvVars.SystemAccessToken} is set but this does not appear to be an Azure DevOps Pipeline environment. "
+                        + "Having this variable set on a developer machine is unusual. It will be ignored.");
+                }
+                else
+                {
+                    logger.LogDebug($"Using PAT from env var {pat.EnvVarSource}");
+                    logger.LogInformation(FormatToken(pat.Value, this.Output, Authorization.Basic));
+                    return 0;
+                }
+            }
+            else if (env.IsAdoPipeline())
+            {
+                // In a pipeline but no token was found at all.
+                logger.LogError(
+                    $"Running in an Azure DevOps Pipeline environment but {EnvVars.SystemAccessToken} is not set. "
+                    + "Interactive authentication is not possible in a pipeline. "
+                    + "Ensure the pipeline has access to the system token.");
+                return 1;
             }
 
             // If command line options for mode are not specified, then use the environment variables.
diff --git a/src/AzureAuth/Commands/Ado/Pat/CommandScopes.cs b/src/AzureAuth/Commands/Ado/Pat/CommandScopes.cs
index bbf0171a..aa9bdb69 100644
--- a/src/AzureAuth/Commands/Ado/Pat/CommandScopes.cs
+++ b/src/AzureAuth/Commands/Ado/Pat/CommandScopes.cs
@@ -10,8 +10,7 @@
 namespace Microsoft.Authentication.AzureAuth.Commands.Ado.Pat
 {
     /// <summary>
-    /// Command to print the list of available scopes
-
+    /// Command to print the list of available scopes for ADO PATs.
     /// </summary>
     [Command("scopes", Description = "List the valid ado pat scopes")]
     public class CommandScopes
diff --git a/src/AzureAuth/EnvVars.cs b/src/AzureAuth/EnvVars.cs
index 4b439e0b..3842c84f 100644
--- a/src/AzureAuth/EnvVars.cs
+++ b/src/AzureAuth/EnvVars.cs
@@ -36,6 +36,12 @@ public static class EnvVars
         /// </summary>
         public static readonly string NoUser = $"{EnvVarPrefix}_NO_USER";
 
+        /// <summary>
+        /// Name of the env var set by Azure DevOps Pipelines to indicate a pipeline environment.
+        /// Value is "True" when running in an ADO Pipeline.
+        /// </summary>
+        public const string TfBuild = "TF_BUILD";
+
         /// <summary>
         /// Name of the env var for the Azure DevOps pipelines default personal access token.
         /// </summary>
diff --git a/src/AzureAuth/IEnvExtensions.cs b/src/AzureAuth/IEnvExtensions.cs
index f67f4143..ad214a6c 100644
--- a/src/AzureAuth/IEnvExtensions.cs
+++ b/src/AzureAuth/IEnvExtensions.cs
@@ -5,7 +5,6 @@ namespace Microsoft.Authentication.AzureAuth
 {
     using Microsoft.Authentication.MSALWrapper;
     using Microsoft.Office.Lasso.Interfaces;
-    using Microsoft.Office.Lasso.Telemetry;
     using System.Collections.Generic;
     using System;
 
@@ -16,6 +15,16 @@ public static class IEnvExtensions
     {
         private const string CorextPositiveValue = "1";
 
+        /// <summary>
+        /// Determines whether we are running in an Azure DevOps Pipeline environment.
+        /// </summary>
+        /// <param name="env">The <see cref="IEnv"/> to use to get environment variables.</param>
+        /// <returns>True if running in an Azure DevOps Pipeline.</returns>
+        public static bool IsAdoPipeline(this IEnv env)
+        {
+            return string.Equals("True", env.Get(EnvVars.TfBuild), StringComparison.OrdinalIgnoreCase);
+        }
+
         /// <summary>
         /// Determines whether interactive auth is disabled or not.
         /// </summary>
@@ -31,7 +40,6 @@ public static bool InteractiveAuthDisabled(this IEnv env)
         /// Get the auth modes from the environment or set the default.
         /// </summary>
         /// <param name="env">The <see cref="IEnv"/> to use.</param>
-        /// <param name="eventData">Event data to add the auth mode to.</param>
         /// <returns>AuthModes.</returns>
         public static IEnumerable<AuthMode> ReadAuthModeFromEnvOrSetDefault(this IEnv env)
         {