diff --git a/CHANGELOG.md b/CHANGELOG.md
index cd5246e..4eed9b7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,12 @@
 - X10D: Added `Color.GetClosestConsoleColor()`
 - X10D: Added `DateTime.GetIso8601WeekOfYear()` and `DateTimeOffset.GetIso8601WeekOfYear()`
 - X10D: Added `DirectoryInfo.Clear([bool])`
+- X10D: Added `IEnumerable<T>.CountWhereNot(Func<T, bool>)`
+- X10D: Added `IEnumerable<T>.FirstWhereNot(Func<T, bool>)`
+- X10D: Added `IEnumerable<T>.FirstWhereNotOrDefault(Func<T, bool>)`
+- X10D: Added `IEnumerable<T>.LastWhereNot(Func<T, bool>)`
+- X10D: Added `IEnumerable<T>.LastWhereNotOrDefault(Func<T, bool>)`
+- X10D: Added `IEnumerable<T>.WhereNot(Func<T, bool>)`
 - X10D: Added `IList<T>.RemoveRange(Range)`
 - X10D: Added `IList<T>.Swap(IList<T>)` (#62)
 - X10D: Added `Point.IsOnLine(LineF)`, `Point.IsOnLine(PointF, PointF)`, and `Point.IsOnLine(Vector2, Vector2)`
diff --git a/X10D.Tests/src/Collections/EnumerableTests.cs b/X10D.Tests/src/Collections/EnumerableTests.cs
index 533a00e..1631040 100644
--- a/X10D.Tests/src/Collections/EnumerableTests.cs
+++ b/X10D.Tests/src/Collections/EnumerableTests.cs
@@ -1,11 +1,40 @@
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using X10D.Collections;
+using X10D.Core;
 
 namespace X10D.Tests.Collections;
 
 [TestClass]
 public class EnumerableTests
 {
+    [TestMethod]
+    public void CountWhereNot_ShouldReturnCorrectCount_GivenSequence()
+    {
+        var enumerable = new[] {2, 4, 6, 7, 8, 9, 10};
+        int count = enumerable.CountWhereNot(x => x % 2 == 0);
+        Assert.AreEqual(2, count);
+    }
+
+    [TestMethod]
+    public void CountWhereNot_ShouldThrowArgumentNullException_GivenNullSource()
+    {
+        Assert.ThrowsException<ArgumentNullException>(() => ((IEnumerable<int>?)null)!.CountWhereNot(x => x % 2 == 0));
+    }
+
+    [TestMethod]
+    public void CountWhereNot_ShouldThrowOverflowException_GivenLargeSource()
+    {
+        IEnumerable<byte> GetValues()
+        {
+            while (true)
+            {
+                yield return 1;
+            }
+        }
+
+        Assert.ThrowsException<OverflowException>(() => GetValues().CountWhereNot(x => x % 2 == 0));
+    }
+
     [TestMethod]
     public void DisposeAll_ShouldDispose_GivenCollection()
     {
@@ -36,6 +65,72 @@ public class EnumerableTests
         await Assert.ThrowsExceptionAsync<ArgumentNullException>(async () => await collection!.DisposeAllAsync());
     }
 
+    [TestMethod]
+    public void FirstWhereNot_ShouldReturnCorrectElements_GivenSequence()
+    {
+        var enumerable = new[] {2, 4, 6, 7, 8, 9, 10};
+        int result = enumerable.FirstWhereNot(x => x % 2 == 0);
+        Assert.AreEqual(7, result);
+    }
+
+    [TestMethod]
+    public void FirstWhereNot_ShouldThrowArgumentNullException_GivenNullSource()
+    {
+        Assert.ThrowsException<ArgumentNullException>(() => ((IEnumerable<int>?)null)!.FirstWhereNot(x => x % 2 == 0));
+    }
+
+    [TestMethod]
+    public void FirstWhereNot_ShouldThrowArgumentNullException_GivenNullPredicate()
+    {
+        Assert.ThrowsException<ArgumentNullException>(() => Array.Empty<int>().FirstWhereNotOrDefault(null!));
+    }
+
+    [TestMethod]
+    public void FirstWhereNot_ShouldThrowInvalidOperationException_GivenEmptySource()
+    {
+        Assert.ThrowsException<InvalidOperationException>(() => Array.Empty<int>().FirstWhereNot(x => x % 2 == 0));
+    }
+
+    [TestMethod]
+    public void FirstWhereNot_ShouldThrowInvalidOperationException_GivenSourceWithNoMatchingElements()
+    {
+        Assert.ThrowsException<InvalidOperationException>(() => 2.AsArrayValue().FirstWhereNot(x => x % 2 == 0));
+    }
+
+    [TestMethod]
+    public void FirstWhereNotOrDefault_ShouldReturnCorrectElements_GivenSequence()
+    {
+        var enumerable = new[] {2, 4, 6, 7, 8, 9, 10};
+        int result = enumerable.FirstWhereNotOrDefault(x => x % 2 == 0);
+        Assert.AreEqual(7, result);
+    }
+
+    [TestMethod]
+    public void FirstWhereNotOrDefault_ShouldThrowArgumentNullException_GivenNullSource()
+    {
+        Assert.ThrowsException<ArgumentNullException>(() => ((IEnumerable<int>?)null)!.FirstWhereNotOrDefault(x => x % 2 == 0));
+    }
+
+    [TestMethod]
+    public void FirstWhereNotOrDefault_ShouldThrowArgumentNullException_GivenNullPredicate()
+    {
+        Assert.ThrowsException<ArgumentNullException>(() => Array.Empty<int>().FirstWhereNotOrDefault(null!));
+    }
+
+    [TestMethod]
+    public void FirstWhereNotOrDefault_ShouldReturnDefault_GivenEmptySource()
+    {
+        int result = Array.Empty<int>().FirstWhereNotOrDefault(x => x % 2 == 0);
+        Assert.AreEqual(default, result);
+    }
+
+    [TestMethod]
+    public void FirstWhereNotOrDefault_ShouldReturnDefault_GivenSourceWithNoMatchingElements()
+    {
+        int result = 2.AsArrayValue().FirstWhereNotOrDefault(x => x % 2 == 0);
+        Assert.AreEqual(default, result);
+    }
+
     [TestMethod]
     public void For_ShouldTransform_GivenTransformationDelegate()
     {
@@ -94,6 +189,72 @@ public class EnumerableTests
         Assert.ThrowsException<ArgumentNullException>(() => source.ForEach(null!));
     }
 
+    [TestMethod]
+    public void LastWhereNot_ShouldReturnCorrectElements_GivenSequence()
+    {
+        var enumerable = new[] {2, 4, 6, 7, 8, 9, 10};
+        int result = enumerable.LastWhereNot(x => x % 2 == 0);
+        Assert.AreEqual(9, result);
+    }
+
+    [TestMethod]
+    public void LastWhereNot_ShouldThrowArgumentNullException_GivenNullSource()
+    {
+        Assert.ThrowsException<ArgumentNullException>(() => ((IEnumerable<int>?)null)!.LastWhereNot(x => x % 2 == 0));
+    }
+
+    [TestMethod]
+    public void LastWhereNot_ShouldThrowArgumentNullException_GivenNullPredicate()
+    {
+        Assert.ThrowsException<ArgumentNullException>(() => Array.Empty<int>().LastWhereNot(null!));
+    }
+
+    [TestMethod]
+    public void LastWhereNot_ShouldThrowInvalidOperationException_GivenEmptySource()
+    {
+        Assert.ThrowsException<InvalidOperationException>(() => Array.Empty<int>().LastWhereNot(x => x % 2 == 0));
+    }
+
+    [TestMethod]
+    public void LastWhereNot_ShouldThrowInvalidOperationException_GivenSourceWithNoMatchingElements()
+    {
+        Assert.ThrowsException<InvalidOperationException>(() => 2.AsArrayValue().LastWhereNot(x => x % 2 == 0));
+    }
+
+    [TestMethod]
+    public void LastWhereNotOrDefault_ShouldReturnCorrectElements_GivenSequence()
+    {
+        var enumerable = new[] {2, 4, 6, 7, 8, 9, 10};
+        int result = enumerable.LastWhereNotOrDefault(x => x % 2 == 0);
+        Assert.AreEqual(9, result);
+    }
+
+    [TestMethod]
+    public void LastWhereNotOrDefault_ShouldThrowArgumentNullException_GivenNullSource()
+    {
+        Assert.ThrowsException<ArgumentNullException>(() => ((IEnumerable<int>?)null)!.LastWhereNotOrDefault(x => x % 2 == 0));
+    }
+
+    [TestMethod]
+    public void LastWhereNotOrDefault_ShouldThrowArgumentNullException_GivenNullPredicate()
+    {
+        Assert.ThrowsException<ArgumentNullException>(() => Array.Empty<int>().LastWhereNotOrDefault(null!));
+    }
+
+    [TestMethod]
+    public void LastWhereNotOrDefault_ShouldReturnDefault_GivenEmptySource()
+    {
+        int result = Array.Empty<int>().LastWhereNotOrDefault(x => x % 2 == 0);
+        Assert.AreEqual(default, result);
+    }
+
+    [TestMethod]
+    public void LastWhereNotOrDefault_ShouldReturnDefault_GivenSourceWithNoMatchingElements()
+    {
+        int result = 2.AsArrayValue().LastWhereNotOrDefault(x => x % 2 == 0);
+        Assert.AreEqual(default, result);
+    }
+
     [TestMethod]
     public void Shuffled_ShouldThrow_GivenNull()
     {
@@ -112,6 +273,20 @@ public class EnumerableTests
         CollectionAssert.AreNotEqual(array, shuffled);
     }
 
+    [TestMethod]
+    public void WhereNot_ShouldReturnCorrectElements_GivenSequence()
+    {
+        var enumerable = new[] {2, 4, 6, 7, 8, 9, 10};
+        IEnumerable<int> result = enumerable.WhereNot(x => x % 2 == 0);
+        CollectionAssert.AreEqual(new[] {7, 9}, result.ToArray());
+    }
+
+    [TestMethod]
+    public void WhereNot_ShouldThrowArgumentNullException_GivenNullSource()
+    {
+        Assert.ThrowsException<ArgumentNullException>(() => ((IEnumerable<int>?)null)!.WhereNot(x => x % 2 == 0));
+    }
+
     private class DummyClass
     {
         public int Value { get; set; }
diff --git a/X10D/src/Collections/EnumerableExtensions.cs b/X10D/src/Collections/EnumerableExtensions.cs
index 4a75370..9747235 100644
--- a/X10D/src/Collections/EnumerableExtensions.cs
+++ b/X10D/src/Collections/EnumerableExtensions.cs
@@ -7,6 +7,108 @@ namespace X10D.Collections;
 /// </summary>
 public static class EnumerableExtensions
 {
+    /// <summary>
+    ///     Returns a number that represents how many elements in the specified sequence do not satisfy a condition.
+    /// </summary>
+    /// <param name="source">A sequence that contains elements to be tested and counted.</param>
+    /// <param name="predicate">A function to test each element for a condition.</param>
+    /// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
+    /// <returns>
+    ///     A number that represents how many elements in the sequence do not satisfy the condition in the
+    ///     <paramref name="predicate" /> function.
+    /// </returns>
+    /// <exception cref="ArgumentNullException"><paramref name="source" /> or <paramref name="predicate" /> is null.</exception>
+    /// <exception cref="OverflowException">
+    ///     The number of elements in <paramref name="source" /> is larger than <see cref="int.MaxValue" />.
+    /// </exception>
+    [Pure]
+    public static int CountWhereNot<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+    {
+#if NET6_0
+        ArgumentNullException.ThrowIfNull(source);
+        ArgumentNullException.ThrowIfNull(predicate);
+#else
+        if (source is null)
+        {
+            throw new ArgumentNullException(nameof(source));
+        }
+
+        if (predicate is null)
+        {
+            throw new ArgumentNullException(nameof(predicate));
+        }
+#endif
+
+        return source.Count(item => !predicate(item));
+    }
+
+    /// <summary>
+    ///     Returns the first element in a sequence that does not satisfy a specified condition.
+    /// </summary>
+    /// <param name="source">An <see cref="IEnumerable{T}" /> to return an element from.</param>
+    /// <param name="predicate">A function to test each element for a condition.</param>
+    /// <typeparam name="TSource">The type of the elements in <paramref name="source" /></typeparam>
+    /// <returns>The first element in the sequence that fails the test in the specified predicate function.</returns>
+    /// <exception cref="ArgumentNullException"><paramref name="source" /> or <paramref name="predicate" /> is null.</exception>
+    /// <exception cref="InvalidOperationException">
+    ///     <para>No element satisfies the condition in predicate.</para>
+    ///      -or-
+    ///     <para>The source sequence is empty.</para>
+    /// </exception>
+    [Pure]
+    public static TSource FirstWhereNot<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+    {
+#if NET6_0
+        ArgumentNullException.ThrowIfNull(source);
+        ArgumentNullException.ThrowIfNull(predicate);
+#else
+        if (source is null)
+        {
+            throw new ArgumentNullException(nameof(source));
+        }
+
+        if (predicate is null)
+        {
+            throw new ArgumentNullException(nameof(predicate));
+        }
+#endif
+
+        return source.First(item => !predicate(item));
+    }
+
+    /// <summary>
+    ///     Returns the first element in a sequence that does not satisfy a specified condition.
+    /// </summary>
+    /// <param name="source">An <see cref="IEnumerable{T}" /> to return an element from.</param>
+    /// <param name="predicate">A function to test each element for a condition.</param>
+    /// <typeparam name="TSource">The type of the elements in <paramref name="source" /></typeparam>
+    /// <returns>
+    ///     <see langword="default(TSource)" /> if <paramref name="source" /> is empty or if no element passes the test specified
+    ///     by <paramref name="predicate"/>; otherwise, the first element in <paramref name="source" /> that fails the test
+    ///     specified by <paramref name="predicate" />.
+    /// </returns>
+    /// <exception cref="ArgumentNullException"><paramref name="source" /> or <paramref name="predicate" /> is null.</exception>
+    [Pure]
+    public static TSource? FirstWhereNotOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+    {
+#if NET6_0
+        ArgumentNullException.ThrowIfNull(source);
+        ArgumentNullException.ThrowIfNull(predicate);
+#else
+        if (source is null)
+        {
+            throw new ArgumentNullException(nameof(source));
+        }
+
+        if (predicate is null)
+        {
+            throw new ArgumentNullException(nameof(predicate));
+        }
+#endif
+
+        return source.FirstOrDefault(item => !predicate(item));
+    }
+
     /// <summary>
     ///     Performs the specified action on each element of the <see cref="IEnumerable{T}" />.
     /// </summary>
@@ -128,6 +230,73 @@ public static class EnumerableExtensions
         }
     }
 
+    /// <summary>
+    ///     Returns the last element in a sequence that does not satisfy a specified condition.
+    /// </summary>
+    /// <param name="source">An <see cref="IEnumerable{T}" /> to return an element from.</param>
+    /// <param name="predicate">A function to test each element for a condition.</param>
+    /// <typeparam name="TSource">The type of the elements in <paramref name="source" /></typeparam>
+    /// <returns>The last element in the sequence that fails the test in the specified predicate function.</returns>
+    /// <exception cref="ArgumentNullException"><paramref name="source" /> or <paramref name="predicate" /> is null.</exception>
+    /// <exception cref="InvalidOperationException">
+    ///     <para>No element satisfies the condition in predicate.</para>
+    ///      -or-
+    ///     <para>The source sequence is empty.</para>
+    /// </exception>
+    [Pure]
+    public static TSource LastWhereNot<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+    {
+#if NET6_0
+        ArgumentNullException.ThrowIfNull(source);
+        ArgumentNullException.ThrowIfNull(predicate);
+#else
+        if (source is null)
+        {
+            throw new ArgumentNullException(nameof(source));
+        }
+
+        if (predicate is null)
+        {
+            throw new ArgumentNullException(nameof(predicate));
+        }
+#endif
+
+        return source.Last(item => !predicate(item));
+    }
+
+    /// <summary>
+    ///     Returns the last element in a sequence that does not satisfy a specified condition.
+    /// </summary>
+    /// <param name="source">An <see cref="IEnumerable{T}" /> to return an element from.</param>
+    /// <param name="predicate">A function to test each element for a condition.</param>
+    /// <typeparam name="TSource">The type of the elements in <paramref name="source" /></typeparam>
+    /// <returns>
+    ///     <see langword="default(TSource)" /> if <paramref name="source" /> is empty or if no element passes the test specified
+    ///     by <paramref name="predicate"/>; otherwise, the last element in <paramref name="source" /> that fails the test
+    ///     specified by <paramref name="predicate" />.
+    /// </returns>
+    /// <exception cref="ArgumentNullException"><paramref name="source" /> or <paramref name="predicate" /> is null.</exception>
+    [Pure]
+    public static TSource? LastWhereNotOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+    {
+#if NET6_0
+        ArgumentNullException.ThrowIfNull(source);
+        ArgumentNullException.ThrowIfNull(predicate);
+#else
+        if (source is null)
+        {
+            throw new ArgumentNullException(nameof(source));
+        }
+
+        if (predicate is null)
+        {
+            throw new ArgumentNullException(nameof(predicate));
+        }
+#endif
+
+        return source.LastOrDefault(item => !predicate(item));
+    }
+
     /// <summary>
     ///     Reorganizes the elements in an enumerable by implementing a Fisher-Yates shuffle, and returns th shuffled result.
     /// </summary>
@@ -152,4 +321,35 @@ public static class EnumerableExtensions
         list.Shuffle(random);
         return list.AsReadOnly();
     }
+
+    /// <summary>
+    ///     Filters a sequence of values based on a predicate, such that all elements in the result do not match the predicate.
+    /// </summary>
+    /// <param name="source">An <see cref="IEnumerable{T}" /> to filter.</param>
+    /// <param name="predicate">A function to test each element for a condition.</param>
+    /// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
+    /// <returns>
+    ///     An <see cref="IEnumerable{T}" /> that contains elements from the input sequence that do not satisfy the condition.
+    /// </returns>
+    /// <exception cref="ArgumentNullException"><paramref name="source" /> or <paramref name="predicate" /> is null.</exception>
+    [Pure]
+    public static IEnumerable<TSource> WhereNot<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+    {
+#if NET6_0
+        ArgumentNullException.ThrowIfNull(source);
+        ArgumentNullException.ThrowIfNull(predicate);
+#else
+        if (source is null)
+        {
+            throw new ArgumentNullException(nameof(source));
+        }
+
+        if (predicate is null)
+        {
+            throw new ArgumentNullException(nameof(predicate));
+        }
+#endif
+
+        return source.Where(item => !predicate(item));
+    }
 }