diff --git a/src/PlaneExtensions.cs b/src/PlaneExtensions.cs
index 3e09c7f..86d4575 100644
--- a/src/PlaneExtensions.cs
+++ b/src/PlaneExtensions.cs
@@ -1,18 +1,32 @@
 namespace Nine.Geometry
 {
     using System;
-    using System.ComponentModel;
     using System.Numerics;
 
-    [EditorBrowsable(EditorBrowsableState.Never)]
     public static class PlaneExtensions
     {
+        /// <summary>
+        /// Returns whether the plane is valid.
+        /// </summary>
+        /// <param name="plane"></param>
+        /// <returns></returns>
+        public static bool IsValid(this Plane plane)
+            => plane.Normal.LengthSquared() > 0.0f;
+
+        /// <summary>
+        /// Returns the <see cref="Plane"/> with a flipped normal.
+        /// </summary>
+        /// <param name="plane"></param>
+        /// <returns></returns>
+        public static Plane Flip(this Plane plane)
+            => new Plane(plane.Normal * -1.0f, plane.D * -1.0f);
+
         public static PlaneIntersectionType Intersects(this Plane plane, Vector3 vector)
         {
             // TODO: Implement
             throw new NotImplementedException();
         }
-        
+
         public static PlaneIntersectionType Intersects(this Plane plane, BoundingBox boundingBox)
         {
             PlaneIntersectionType result;
diff --git a/test/PlaneExtensionsTest.cs b/test/PlaneExtensionsTest.cs
new file mode 100644
index 0000000..29bad91
--- /dev/null
+++ b/test/PlaneExtensionsTest.cs
@@ -0,0 +1,52 @@
+namespace Nine.Geometry.Test
+{
+    using System.Numerics;
+    using Xunit;
+
+    public class PlaneExtensionsTest
+    {
+        [Fact]
+        public void IsValid()
+        {
+            var plane = Plane.CreateFromVertices(
+                new Vector3(0, 0, 0),
+                new Vector3(0, 1, 0),
+                new Vector3(1, 1, 0)
+            );
+
+            var valid = plane.IsValid();
+
+            Assert.True(valid);
+        }
+
+        [Fact]
+        public void IsValid_False()
+        {
+            var plane = Plane.CreateFromVertices(
+                new Vector3(0, 0, 0),
+                new Vector3(0, 0, 0),
+                new Vector3(0, 0, 0)
+            );
+
+            var valid = plane.IsValid();
+
+            Assert.False(valid);
+        }
+
+        [Fact]
+        public void Flip()
+        {
+            var plane = Plane.CreateFromVertices(
+                new Vector3(0, 0, 0),
+                new Vector3(0, 1, 0),
+                new Vector3(1, 1, 0)
+            );
+
+            var org_normal = plane.Normal;
+            var new_normal = plane.Flip().Normal;
+
+            Assert.Equal(new Vector3(0, 0, -1), org_normal);
+            Assert.Equal(new Vector3(0, 0, 1), new_normal);
+        }
+    }
+}