diff --git a/src/Polly/AsyncPolicy.TResult.cs b/src/Polly/AsyncPolicy.TResult.cs
index b8dfbd4555a..e6c8898a168 100644
--- a/src/Polly/AsyncPolicy.TResult.cs
+++ b/src/Polly/AsyncPolicy.TResult.cs
@@ -1,4 +1,5 @@
-namespace Polly;
+#nullable enable
+namespace Polly;
 
 /// <summary>
 /// Transient exception handling policies that can be applied to asynchronous delegates
@@ -22,7 +23,7 @@ internal AsyncPolicy(
     /// Constructs a new instance of a derived <see cref="AsyncPolicy{TResult}"/> type with the passed <paramref name="policyBuilder"/>.
     /// </summary>
     /// <param name="policyBuilder">A <see cref="PolicyBuilder{TResult}"/> indicating which exceptions and results the policy should handle.</param>
-    protected AsyncPolicy(PolicyBuilder<TResult> policyBuilder = null)
+    protected AsyncPolicy(PolicyBuilder<TResult>? policyBuilder = null)
         : base(policyBuilder)
     {
     }
diff --git a/src/Polly/AsyncPolicy.cs b/src/Polly/AsyncPolicy.cs
index 50d5b910ae3..e2a37c717c3 100644
--- a/src/Polly/AsyncPolicy.cs
+++ b/src/Polly/AsyncPolicy.cs
@@ -1,4 +1,5 @@
-namespace Polly;
+#nullable enable
+namespace Polly;
 
 /// <summary>
 /// Transient exception handling policies that can be applied to asynchronous delegates
@@ -18,7 +19,7 @@ internal AsyncPolicy(ExceptionPredicates exceptionPredicates)
     /// Constructs a new instance of a derived <see cref="AsyncPolicy"/> type with the passed <paramref name="policyBuilder"/>.
     /// </summary>
     /// <param name="policyBuilder">A <see cref="PolicyBuilder"/> specifying which exceptions the policy should handle. </param>
-    protected AsyncPolicy(PolicyBuilder policyBuilder = null)
+    protected AsyncPolicy(PolicyBuilder? policyBuilder = null)
         : base(policyBuilder)
     {
     }
diff --git a/src/Polly/Caching/AsyncCacheTResultSyntax.cs b/src/Polly/Caching/AsyncCacheTResultSyntax.cs
index 6faad6d8c7c..1f76748397d 100644
--- a/src/Polly/Caching/AsyncCacheTResultSyntax.cs
+++ b/src/Polly/Caching/AsyncCacheTResultSyntax.cs
@@ -1,4 +1,5 @@
-namespace Polly;
+#nullable enable
+namespace Polly;
 
 public partial class Policy
 {
@@ -13,7 +14,7 @@ public partial class Policy
     /// <param name="onCacheError">Delegate to call if an exception is thrown when attempting to get a value from or put a value into the cache, passing the execution context, the cache key, and the exception.</param>
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider cacheProvider, TimeSpan ttl, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider cacheProvider, TimeSpan ttl, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
 
@@ -32,7 +33,7 @@ public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
 
@@ -52,7 +53,7 @@ public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider cacheProvider, TimeSpan ttl, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider cacheProvider, TimeSpan ttl, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
 
@@ -73,7 +74,7 @@ public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
 
@@ -93,7 +94,7 @@ public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider cacheProvider, TimeSpan ttl, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider cacheProvider, TimeSpan ttl, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
 
@@ -114,7 +115,7 @@ public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
 
@@ -351,7 +352,7 @@ public static AsyncCachePolicy<TResult> CacheAsync<TResult>(
     /// <param name="onCacheError">Delegate to call if an exception is thrown when attempting to get a value from or put a value into the cache, passing the execution context, the cache key, and the exception.</param>
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, TimeSpan ttl, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, TimeSpan ttl, Action<Context, string, Exception>? onCacheError = null)
         => CacheAsync<TResult>(cacheProvider, new RelativeTtl(ttl), DefaultCacheKeyStrategy.Instance.GetCacheKey, onCacheError);
 
     /// <summary>
@@ -366,7 +367,7 @@ public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, ITtlStrategy ttlStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, ITtlStrategy ttlStrategy, Action<Context, string, Exception>? onCacheError = null)
         => CacheAsync<TResult>(cacheProvider, ttlStrategy, DefaultCacheKeyStrategy.Instance.GetCacheKey, onCacheError);
 
     /// <summary>
@@ -381,7 +382,7 @@ public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, ITtlStrategy<TResult> ttlStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, ITtlStrategy<TResult> ttlStrategy, Action<Context, string, Exception>? onCacheError = null)
         => CacheAsync<TResult>(cacheProvider, ttlStrategy, DefaultCacheKeyStrategy.Instance.GetCacheKey, onCacheError);
 
     /// <summary>
@@ -397,7 +398,7 @@ public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, TimeSpan ttl, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, TimeSpan ttl, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
         => CacheAsync<TResult>(cacheProvider, new RelativeTtl(ttl), cacheKeyStrategy.GetCacheKey, onCacheError);
 
     /// <summary>
@@ -414,7 +415,7 @@ public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, ITtlStrategy ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, ITtlStrategy ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         onCacheError = onCacheError ?? ((_, _, _) => { });
 
@@ -437,7 +438,7 @@ public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, ITtlStrategy<TResult> ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, ITtlStrategy<TResult> ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         onCacheError = onCacheError ?? ((_, _, _) => { });
 
@@ -459,7 +460,7 @@ public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, TimeSpan ttl, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, TimeSpan ttl, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
         => CacheAsync<TResult>(cacheProvider, new RelativeTtl(ttl), cacheKeyStrategy, onCacheError);
 
     /// <summary>
@@ -476,7 +477,7 @@ public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, ITtlStrategy ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, ITtlStrategy ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         onCacheError = onCacheError ?? ((_, _, _) => { });
 
@@ -499,7 +500,7 @@ public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, ITtlStrategy<TResult> ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider<TResult> cacheProvider, ITtlStrategy<TResult> ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         onCacheError = onCacheError ?? ((_, _, _) => { });
 
diff --git a/src/Polly/Policy.TResult.cs b/src/Polly/Policy.TResult.cs
index a6512eaafcd..8f52e3a1cc9 100644
--- a/src/Polly/Policy.TResult.cs
+++ b/src/Polly/Policy.TResult.cs
@@ -1,4 +1,5 @@
-namespace Polly;
+#nullable enable
+namespace Polly;
 
 /// <summary>
 /// Transient fault handling policies that can be applied to delegates returning results of type <typeparamref name="TResult"/>
@@ -19,7 +20,7 @@ internal Policy(ExceptionPredicates exceptionPredicates, ResultPredicates<TResul
     /// Constructs a new instance of a derived <see cref="Policy{TResult}"/> type with the passed <paramref name="policyBuilder"/>.
     /// </summary>
     /// <param name="policyBuilder">A <see cref="PolicyBuilder{TResult}"/> indicating which exceptions and results the policy should handle.</param>
-    protected Policy(PolicyBuilder<TResult> policyBuilder = null)
+    protected Policy(PolicyBuilder<TResult>? policyBuilder = null)
         : base(policyBuilder)
     {
     }
diff --git a/src/Polly/Policy.cs b/src/Polly/Policy.cs
index 448defb34c1..7d8a02ac1ba 100644
--- a/src/Polly/Policy.cs
+++ b/src/Polly/Policy.cs
@@ -1,4 +1,5 @@
-namespace Polly;
+#nullable enable
+namespace Polly;
 
 /// <summary>
 /// Transient exception handling policies that can be applied to synchronous delegates
@@ -18,7 +19,7 @@ internal Policy(ExceptionPredicates exceptionPredicates)
     /// Constructs a new instance of a derived <see cref="Policy"/> type with the passed <paramref name="policyBuilder"/>.
     /// </summary>
     /// <param name="policyBuilder">A <see cref="PolicyBuilder"/> specifying which exceptions the policy should handle. </param>
-    protected Policy(PolicyBuilder policyBuilder = null)
+    protected Policy(PolicyBuilder? policyBuilder = null)
         : base(policyBuilder)
     {
     }
diff --git a/src/Polly/Retry/AsyncRetryEngine.cs b/src/Polly/Retry/AsyncRetryEngine.cs
index 8eb287e6e6f..57ae503de15 100644
--- a/src/Polly/Retry/AsyncRetryEngine.cs
+++ b/src/Polly/Retry/AsyncRetryEngine.cs
@@ -1,4 +1,5 @@
-namespace Polly.Retry;
+#nullable enable
+namespace Polly.Retry;
 
 internal static class AsyncRetryEngine
 {
@@ -10,12 +11,12 @@ internal static async Task<TResult> ImplementationAsync<TResult>(
         ResultPredicates<TResult> shouldRetryResultPredicates,
         Func<DelegateResult<TResult>, TimeSpan, int, Context, Task> onRetryAsync,
         int permittedRetryCount = Int32.MaxValue,
-        IEnumerable<TimeSpan> sleepDurationsEnumerable = null,
-        Func<int, DelegateResult<TResult>, Context, TimeSpan> sleepDurationProvider = null,
+        IEnumerable<TimeSpan>? sleepDurationsEnumerable = null,
+        Func<int, DelegateResult<TResult>, Context, TimeSpan>? sleepDurationProvider = null,
         bool continueOnCapturedContext = false)
     {
         int tryCount = 0;
-        IEnumerator<TimeSpan> sleepDurationsEnumerator = sleepDurationsEnumerable?.GetEnumerator();
+        IEnumerator<TimeSpan>? sleepDurationsEnumerator = sleepDurationsEnumerable?.GetEnumerator();
 
         try
         {