diff --git a/samples/IdentitySample.Mvc/Controllers/ManageController.cs b/samples/IdentitySample.Mvc/Controllers/ManageController.cs
index 13223ceed9..f33dfe020a 100644
--- a/samples/IdentitySample.Mvc/Controllers/ManageController.cs
+++ b/samples/IdentitySample.Mvc/Controllers/ManageController.cs
@@ -1,5 +1,5 @@
 using System.Linq;
-using System.Security.Principal;
+using System.Security.Claims;
 using System.Threading.Tasks;
 using IdentitySample.Models;
 using IdentitySamples;
diff --git a/src/Microsoft.AspNet.Identity/PrincipalExtensions.cs b/src/Microsoft.AspNet.Identity/PrincipalExtensions.cs
index 669af4cfa3..6aaf667d4c 100644
--- a/src/Microsoft.AspNet.Identity/PrincipalExtensions.cs
+++ b/src/Microsoft.AspNet.Identity/PrincipalExtensions.cs
@@ -2,62 +2,58 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System.Linq;
-using System.Security.Claims;
 using Microsoft.AspNet.Identity;
 
-namespace System.Security.Principal
+namespace System.Security.Claims
 {
     /// <summary>
-    /// Claims related extensions for <see cref="IPrincipal"/>.
+    /// Claims related extensions for <see cref="ClaimsPrincipal"/>.
     /// </summary>
     public static class PrincipalExtensions
     {
         /// <summary>
         /// Returns the Name claim value if present otherwise returns null.
         /// </summary>
-        /// <param name="principal">The <see cref="IPrincipal"/> instance this method extends.</param>
+        /// <param name="principal">The <see cref="ClaimsPrincipal"/> instance this method extends.</param>
         /// <returns>The Name claim value, or null if the claim is not present.</returns>
         /// <remarks>The name claim is identified by <see cref="ClaimsIdentity.DefaultNameClaimType"/>.</remarks>
-        public static string GetUserName(this IPrincipal principal)
+        public static string GetUserName(this ClaimsPrincipal principal)
         {
             if (principal == null)
             {
                 throw new ArgumentNullException(nameof(principal));
             }
-            var cp = principal as ClaimsPrincipal;
-            return cp != null ? cp.FindFirstValue(ClaimsIdentity.DefaultNameClaimType) : null;
+            return principal.FindFirstValue(ClaimsIdentity.DefaultNameClaimType);
         }
 
         /// <summary>
         /// Returns the User ID claim value if present otherwise returns null.
         /// </summary>
-        /// <param name="principal">The <see cref="IPrincipal"/> instance this method extends.</param>
+        /// <param name="principal">The <see cref="ClaimsPrincipal"/> instance this method extends.</param>
         /// <returns>The User ID claim value, or null if the claim is not present.</returns>
         /// <remarks>The name claim is identified by <see cref="ClaimTypes.NameIdentifier"/>.</remarks>
-        public static string GetUserId(this IPrincipal principal)
+        public static string GetUserId(this ClaimsPrincipal principal)
         {
             if (principal == null)
             {
                 throw new ArgumentNullException(nameof(principal));
             }
-            var ci = principal as ClaimsPrincipal;
-            return ci != null ? ci.FindFirstValue(ClaimTypes.NameIdentifier) : null;
+            return principal.FindFirstValue(ClaimTypes.NameIdentifier);
         }
 
         /// <summary>
         /// Returns true if the principal has an identity with the application cookie identity
         /// </summary>
-        /// <param name="principal">The <see cref="IPrincipal"/> instance this method extends.</param>
+        /// <param name="principal">The <see cref="ClaimsPrincipal"/> instance this method extends.</param>
         /// <returns>True if the user is logged in with identity.</returns>
-        public static bool IsSignedIn(this IPrincipal principal)
+        public static bool IsSignedIn(this ClaimsPrincipal principal)
         {
             if (principal == null)
             {
                 throw new ArgumentNullException(nameof(principal));
             }
-            var p = principal as ClaimsPrincipal;
-            return p?.Identities != null && 
-                p.Identities.Any(i => i.AuthenticationType == IdentityOptions.ApplicationCookieAuthenticationType);
+            return principal?.Identities != null && 
+                principal.Identities.Any(i => i.AuthenticationType == IdentityOptions.ApplicationCookieAuthenticationType);
         }
 
         /// <summary>
diff --git a/src/Microsoft.AspNet.Identity/SecurityStampValidator.cs b/src/Microsoft.AspNet.Identity/SecurityStampValidator.cs
index f8ba4b9e0b..12e50cf227 100644
--- a/src/Microsoft.AspNet.Identity/SecurityStampValidator.cs
+++ b/src/Microsoft.AspNet.Identity/SecurityStampValidator.cs
@@ -2,7 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
-using System.Security.Principal;
+using System.Security.Claims;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Authentication.Cookies;
 using Microsoft.Framework.DependencyInjection;
diff --git a/test/Microsoft.AspNet.Identity.Test/PrincipalExtensionsTest.cs b/test/Microsoft.AspNet.Identity.Test/PrincipalExtensionsTest.cs
index 8a2e5b3a47..ce56993d13 100644
--- a/test/Microsoft.AspNet.Identity.Test/PrincipalExtensionsTest.cs
+++ b/test/Microsoft.AspNet.Identity.Test/PrincipalExtensionsTest.cs
@@ -3,7 +3,6 @@
 
 using System;
 using System.Security.Claims;
-using System.Security.Principal;
 using Xunit;
 
 namespace Microsoft.AspNet.Identity.Test
@@ -15,19 +14,10 @@ namespace Microsoft.AspNet.Identity.Test
         [Fact]
         public void IdentityNullCheckTest()
         {
-            IPrincipal p = null;
+            ClaimsPrincipal p = null;
             Assert.Throws<ArgumentNullException>("principal", () => p.GetUserId());
             Assert.Throws<ArgumentNullException>("principal", () => p.GetUserName());
-            ClaimsPrincipal cp = null;
-            Assert.Throws<ArgumentNullException>("principal", () => cp.FindFirstValue(null));
-        }
-
-        [Fact]
-        public void IdentityNullIfNotClaimsIdentityTest()
-        {
-            IPrincipal identity = new TestPrincipal();
-            Assert.Null(identity.GetUserId());
-            Assert.Null(identity.GetUserName());
+            Assert.Throws<ArgumentNullException>("principal", () => p.FindFirstValue(null));
         }
 
         [Fact]
@@ -82,39 +72,5 @@ namespace Microsoft.AspNet.Identity.Test
                 },
                 ExternalAuthenticationScheme));
         }
-
-        private class TestPrincipal : IPrincipal
-        {
-            public IIdentity Identity
-            {
-                get
-                {
-                    throw new NotImplementedException();
-                }
-            }
-
-            public bool IsInRole(string role)
-            {
-                throw new NotImplementedException();
-            }
-        }
-
-        private class TestIdentity : IIdentity
-        {
-            public string AuthenticationType
-            {
-                get { throw new NotImplementedException(); }
-            }
-
-            public bool IsAuthenticated
-            {
-                get { throw new NotImplementedException(); }
-            }
-
-            public string Name
-            {
-                get { throw new NotImplementedException(); }
-            }
-        }
     }
 }
\ No newline at end of file