diff --git a/BitFaster.Caching.UnitTests/BitFaster.Caching.UnitTests.csproj b/BitFaster.Caching.UnitTests/BitFaster.Caching.UnitTests.csproj
index b5aef909..401c9617 100644
--- a/BitFaster.Caching.UnitTests/BitFaster.Caching.UnitTests.csproj
+++ b/BitFaster.Caching.UnitTests/BitFaster.Caching.UnitTests.csproj
@@ -2,7 +2,8 @@
 
   <PropertyGroup>
     <TargetFrameworks>net48;netcoreapp3.1;net6.0;net9.0</TargetFrameworks>
-    <LangVersion>10.0</LangVersion>  
+    <LangVersion Condition="'$(TargetFramework)' == 'net9.0'">13.0</LangVersion>
+    <LangVersion Condition="'$(TargetFramework)' != 'net9.0'">10.0</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>
@@ -30,4 +31,4 @@
     <ProjectReference Include="..\BitFaster.Caching\BitFaster.Caching.csproj" />
   </ItemGroup>
 
-</Project>
\ No newline at end of file
+</Project>
diff --git a/BitFaster.Caching.UnitTests/Lru/ConcurrentLruAlternateLookupTests.cs b/BitFaster.Caching.UnitTests/Lru/ConcurrentLruAlternateLookupTests.cs
new file mode 100644
index 00000000..c274cd17
--- /dev/null
+++ b/BitFaster.Caching.UnitTests/Lru/ConcurrentLruAlternateLookupTests.cs
@@ -0,0 +1,76 @@
+#if NET9_0_OR_GREATER
+using System;
+using BitFaster.Caching.Lru;
+using FluentAssertions;
+using Xunit;
+
+namespace BitFaster.Caching.UnitTests.Lru
+{
+    public class ConcurrentLruAlternateLookupTests
+    {
+        [Fact]
+        public void TryGetAlternateLookupReturnsLookupForCompatibleComparer()
+        {
+            var cache = new ConcurrentLru<string, string>(1, 3, StringComparer.Ordinal);
+            cache.GetOrAdd("42", _ => "value");
+            ReadOnlySpan<char> key = "42";
+
+            cache.TryGetAlternateLookup<ReadOnlySpan<char>>(out var alternate).Should().BeTrue();
+            alternate.TryGet(key, out var value).Should().BeTrue();
+            value.Should().Be("value");
+        }
+
+        [Fact]
+        public void GetAlternateLookupThrowsForIncompatibleComparer()
+        {
+            var cache = new ConcurrentLru<string, string>(1, 3, StringComparer.Ordinal);
+
+            Action act = () => cache.GetAlternateLookup<int>();
+
+            act.Should().Throw<InvalidOperationException>().WithMessage("Incompatible comparer");
+            cache.TryGetAlternateLookup<int>(out var alternate).Should().BeFalse();
+            alternate.Should().BeNull();
+        }
+
+        [Fact]
+        public void AlternateLookupTryRemoveReturnsActualKeyAndValue()
+        {
+            var cache = new ConcurrentLru<string, string>(1, 3, StringComparer.Ordinal);
+            cache.GetOrAdd("42", _ => "value");
+            var alternate = cache.GetAlternateLookup<ReadOnlySpan<char>>();
+            ReadOnlySpan<char> key = "42";
+
+            alternate.TryRemove(key, out var actualKey, out var value).Should().BeTrue();
+
+            actualKey.Should().Be("42");
+            value.Should().Be("value");
+            cache.TryGet("42", out _).Should().BeFalse();
+        }
+
+        [Fact]
+        public void AlternateLookupGetOrAddUsesAlternateKeyOnMissAndHit()
+        {
+            var cache = new ConcurrentLru<string, string>(1, 3, StringComparer.Ordinal);
+            var alternate = cache.GetAlternateLookup<ReadOnlySpan<char>>();
+            var factoryCalls = 0;
+            ReadOnlySpan<char> key = "42";
+
+            alternate.GetOrAdd(key, key =>
+            {
+                factoryCalls++;
+                return $"value-{key.ToString()}";
+            }).Should().Be("value-42");
+
+            alternate.GetOrAdd(key, (_, prefix) =>
+            {
+                factoryCalls++;
+                return prefix;
+            }, "unused").Should().Be("value-42");
+
+            factoryCalls.Should().Be(1);
+            cache.TryGet("42", out var value).Should().BeTrue();
+            value.Should().Be("value-42");
+        }
+    }
+}
+#endif
diff --git a/BitFaster.Caching/CollectionExtensions.cs b/BitFaster.Caching/CollectionExtensions.cs
new file mode 100644
index 00000000..808b5479
--- /dev/null
+++ b/BitFaster.Caching/CollectionExtensions.cs
@@ -0,0 +1,29 @@
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+
+namespace BitFaster.Caching
+{
+    internal static class CollectionExtensions
+    {
+#if NET9_0_OR_GREATER
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static bool IsCompatibleKey<TAlternateKey, TKey, TValue>(this ConcurrentDictionary<TKey, TValue> dictionary)
+            where TAlternateKey : notnull, allows ref struct
+            where TKey : notnull
+        {
+            return dictionary.Comparer is IAlternateEqualityComparer<TAlternateKey, TKey>;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static IAlternateEqualityComparer<TAlternateKey, TKey> GetAlternateComparer<TAlternateKey, TKey, TValue>(this ConcurrentDictionary<TKey, TValue> dictionary)
+            where TAlternateKey : notnull, allows ref struct
+            where TKey : notnull
+        {
+            Debug.Assert(dictionary.IsCompatibleKey<TAlternateKey, TKey, TValue>());
+            return Unsafe.As<IAlternateEqualityComparer<TAlternateKey, TKey>>(dictionary.Comparer!);
+        }
+#endif
+    }
+}
diff --git a/BitFaster.Caching/IAlternateLookup.cs b/BitFaster.Caching/IAlternateLookup.cs
new file mode 100644
index 00000000..47ca7dc2
--- /dev/null
+++ b/BitFaster.Caching/IAlternateLookup.cs
@@ -0,0 +1,53 @@
+#if NET9_0_OR_GREATER
+using System;
+using System.Diagnostics.CodeAnalysis;
+
+namespace BitFaster.Caching
+{
+    /// <summary>
+    /// Provides an alternate-key lookup over a cache.
+    /// </summary>
+    /// <typeparam name="TAlternateKey">The alternate key type.</typeparam>
+    /// <typeparam name="TKey">The cache key type.</typeparam>
+    /// <typeparam name="TValue">The cache value type.</typeparam>
+    public interface IAlternateLookup<TAlternateKey, TKey, TValue>
+        where TAlternateKey : notnull, allows ref struct
+        where TKey : notnull
+    {
+        /// <summary>
+        /// Attempts to get a value using an alternate key.
+        /// </summary>
+        /// <param name="key">The alternate key.</param>
+        /// <param name="value">The cached value when found.</param>
+        /// <returns><see langword="true" /> when the key is found; otherwise, <see langword="false" />.</returns>
+        bool TryGet(TAlternateKey key, [MaybeNullWhen(false)] out TValue value);
+
+        /// <summary>
+        /// Attempts to remove a value using an alternate key.
+        /// </summary>
+        /// <param name="key">The alternate key.</param>
+        /// <param name="actualKey">The removed cache key.</param>
+        /// <param name="value">The removed value.</param>
+        /// <returns><see langword="true" /> when the key is found; otherwise, <see langword="false" />.</returns>
+        bool TryRemove(TAlternateKey key, [MaybeNullWhen(false)] out TKey actualKey, [MaybeNullWhen(false)] out TValue value);
+
+        /// <summary>
+        /// Gets an existing value or adds a new value using an alternate key.
+        /// </summary>
+        /// <param name="key">The alternate key.</param>
+        /// <param name="valueFactory">The value factory.</param>
+        /// <returns>The cached value.</returns>
+        TValue GetOrAdd(TAlternateKey key, Func<TAlternateKey, TValue> valueFactory);
+
+        /// <summary>
+        /// Gets an existing value or adds a new value using an alternate key and factory argument.
+        /// </summary>
+        /// <typeparam name="TArg">The factory argument type.</typeparam>
+        /// <param name="key">The alternate key.</param>
+        /// <param name="valueFactory">The value factory.</param>
+        /// <param name="factoryArgument">The factory argument.</param>
+        /// <returns>The cached value.</returns>
+        TValue GetOrAdd<TArg>(TAlternateKey key, Func<TAlternateKey, TArg, TValue> valueFactory, TArg factoryArgument);
+    }
+}
+#endif
diff --git a/BitFaster.Caching/Lru/ConcurrentLruCore.cs b/BitFaster.Caching/Lru/ConcurrentLruCore.cs
index 76f979f6..22e6954b 100644
--- a/BitFaster.Caching/Lru/ConcurrentLruCore.cs
+++ b/BitFaster.Caching/Lru/ConcurrentLruCore.cs
@@ -882,21 +882,139 @@ private static Optional<ICacheMetrics> CreateMetrics(ConcurrentLruCore<K, V, I,
             return new(new Proxy(lru));
         }
 
-        private static Optional<ICacheEvents<K, V>> CreateEvents(ConcurrentLruCore<K, V, I, P, T> lru)
-        {
-            if (typeof(T) == typeof(NoTelemetryPolicy<K, V>))
-            {
-                return Optional<ICacheEvents<K, V>>.None();
-            }
-
-            return new(new Proxy(lru));
-        }
-
-        // To get JIT optimizations, policies must be structs.
-        // If the structs are returned directly via properties, they will be copied. Since  
-        // telemetryPolicy is a mutable struct, copy is bad. One workaround is to store the 
-        // state within the struct in an object. Since the struct points to the same object
-        // it becomes immutable. However, this object is then somewhere else on the 
+        private static Optional<ICacheEvents<K, V>> CreateEvents(ConcurrentLruCore<K, V, I, P, T> lru)
+        {
+            if (typeof(T) == typeof(NoTelemetryPolicy<K, V>))
+            {
+                return Optional<ICacheEvents<K, V>>.None();
+            }
+
+            return new(new Proxy(lru));
+        }
+
+#if NET9_0_OR_GREATER
+        /// <summary>
+        /// Gets an alternate lookup that can use an alternate key type with the configured comparer.
+        /// </summary>
+        /// <typeparam name="TAlternateKey">The alternate key type.</typeparam>
+        /// <returns>An alternate lookup.</returns>
+        /// <exception cref="InvalidOperationException">The configured comparer does not support <typeparamref name="TAlternateKey" />.</exception>
+        public IAlternateLookup<TAlternateKey, K, V> GetAlternateLookup<TAlternateKey>()
+            where TAlternateKey : notnull, allows ref struct
+        {
+            if (!this.dictionary.IsCompatibleKey<TAlternateKey, K, I>())
+            {
+                Throw.IncompatibleComparer();
+            }
+
+            return new AlternateLookup<TAlternateKey>(this);
+        }
+
+        /// <summary>
+        /// Attempts to get an alternate lookup that can use an alternate key type with the configured comparer.
+        /// </summary>
+        /// <typeparam name="TAlternateKey">The alternate key type.</typeparam>
+        /// <param name="lookup">The alternate lookup when available.</param>
+        /// <returns><see langword="true" /> when the configured comparer supports <typeparamref name="TAlternateKey" />; otherwise, <see langword="false" />.</returns>
+        public bool TryGetAlternateLookup<TAlternateKey>([MaybeNullWhen(false)] out IAlternateLookup<TAlternateKey, K, V> lookup)
+            where TAlternateKey : notnull, allows ref struct
+        {
+            if (this.dictionary.IsCompatibleKey<TAlternateKey, K, I>())
+            {
+                lookup = new AlternateLookup<TAlternateKey>(this);
+                return true;
+            }
+
+            lookup = default;
+            return false;
+        }
+
+        internal readonly struct AlternateLookup<TAlternateKey> : IAlternateLookup<TAlternateKey, K, V>
+            where TAlternateKey : notnull, allows ref struct
+        {
+            internal AlternateLookup(ConcurrentLruCore<K, V, I, P, T> lru)
+            {
+                Debug.Assert(lru is not null);
+                Debug.Assert(lru.dictionary.IsCompatibleKey<TAlternateKey, K, I>());
+                this.Lru = lru;
+                this.Alternate = lru.dictionary.GetAlternateLookup<TAlternateKey>();
+            }
+
+            internal ConcurrentLruCore<K, V, I, P, T> Lru { get; }
+
+            internal ConcurrentDictionary<K, I>.AlternateLookup<TAlternateKey> Alternate { get; }
+
+            public bool TryGet(TAlternateKey key, [MaybeNullWhen(false)] out V value)
+            {
+                if (this.Alternate.TryGetValue(key, out var item))
+                {
+                    return this.Lru.GetOrDiscard(item, out value);
+                }
+
+                value = default;
+                this.Lru.telemetryPolicy.IncrementMiss();
+                return false;
+            }
+
+            public bool TryRemove(TAlternateKey key, [MaybeNullWhen(false)] out K actualKey, [MaybeNullWhen(false)] out V value)
+            {
+                if (this.Alternate.TryRemove(key, out actualKey, out var item))
+                {
+                    this.Lru.OnRemove(actualKey, item, ItemRemovedReason.Removed);
+                    value = item.Value;
+                    return true;
+                }
+
+                actualKey = default;
+                value = default;
+                return false;
+            }
+
+            public V GetOrAdd(TAlternateKey key, Func<TAlternateKey, V> valueFactory)
+            {
+                while (true)
+                {
+                    if (this.TryGet(key, out var value))
+                    {
+                        return value;
+                    }
+
+                    K actualKey = this.Lru.dictionary.GetAlternateComparer<TAlternateKey, K, I>().Create(key);
+
+                    value = valueFactory(key);
+                    if (this.Lru.TryAdd(actualKey, value))
+                    {
+                        return value;
+                    }
+                }
+            }
+
+            public V GetOrAdd<TArg>(TAlternateKey key, Func<TAlternateKey, TArg, V> valueFactory, TArg factoryArgument)
+            {
+                while (true)
+                {
+                    if (this.TryGet(key, out var value))
+                    {
+                        return value;
+                    }
+
+                    K actualKey = this.Lru.dictionary.GetAlternateComparer<TAlternateKey, K, I>().Create(key);
+
+                    value = valueFactory(key, factoryArgument);
+                    if (this.Lru.TryAdd(actualKey, value))
+                    {
+                        return value;
+                    }
+                }
+            }
+        }
+#endif
+
+        // To get JIT optimizations, policies must be structs.
+        // If the structs are returned directly via properties, they will be copied. Since  
+        // telemetryPolicy is a mutable struct, copy is bad. One workaround is to store the 
+        // state within the struct in an object. Since the struct points to the same object
+        // it becomes immutable. However, this object is then somewhere else on the 
         // heap, which slows down the policies with hit counter logic in benchmarks. Likely
         // this approach keeps the structs data members in the same CPU cache line as the LRU.
         // backcompat: remove conditional compile
diff --git a/BitFaster.Caching/Throw.cs b/BitFaster.Caching/Throw.cs
index 99485767..3e1de3ed 100644
--- a/BitFaster.Caching/Throw.cs
+++ b/BitFaster.Caching/Throw.cs
@@ -23,11 +23,14 @@ internal static class Throw
         [DoesNotReturn]
         public static void ScopedRetryFailure() => throw CreateScopedRetryFailure();
 
-        [DoesNotReturn]
-        public static void Disposed<T>() => throw CreateObjectDisposedException<T>();
-
-        [MethodImpl(MethodImplOptions.NoInlining)]
-        private static ArgumentNullException CreateArgumentNullException(ExceptionArgument arg) => new ArgumentNullException(GetArgumentString(arg));
+        [DoesNotReturn]
+        public static void Disposed<T>() => throw CreateObjectDisposedException<T>();
+
+        [DoesNotReturn]
+        public static void IncompatibleComparer() => throw new InvalidOperationException("Incompatible comparer");
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        private static ArgumentNullException CreateArgumentNullException(ExceptionArgument arg) => new ArgumentNullException(GetArgumentString(arg));
 
         [MethodImpl(MethodImplOptions.NoInlining)]
         private static ArgumentOutOfRangeException CreateArgumentOutOfRangeException(string paramName) => new ArgumentOutOfRangeException(paramName);