diff --git a/PolyShim.Tests/NetCore20/StringTests.cs b/PolyShim.Tests/NetCore20/StringTests.cs
index ef40461..b7c5715 100644
--- a/PolyShim.Tests/NetCore20/StringTests.cs
+++ b/PolyShim.Tests/NetCore20/StringTests.cs
@@ -129,4 +129,46 @@ public void Split_String_Count_Test()
         str.Split(" ", 2).Should().Equal("a", "b c");
         str.Split(" ", 1).Should().Equal("a b c");
     }
+
+    [Fact]
+    public void Join_CharSeparator_ObjectArray_Test()
+    {
+        // Act & assert
+        string.Join(',', new object?[] { 1, "two", 3.0 }).Should().Be("1,two,3");
+        string.Join(',', new object?[] { }).Should().Be("");
+        string.Join(',', new object?[] { null, "a", null }).Should().Be(",a,");
+    }
+
+    [Fact]
+    public void Join_CharSeparator_StringArray_Test()
+    {
+        // Act & assert
+        string.Join(',', "a", "b", "c").Should().Be("a,b,c");
+        string.Join(',', new string?[] { }).Should().Be("");
+        string.Join(',', new string?[] { null, "a", null }).Should().Be(",a,");
+    }
+
+    [Fact]
+    public void Join_CharSeparator_EnumerableT_Test()
+    {
+        // Act & assert
+        string.Join(',', new System.Collections.Generic.List<int> { 1, 2, 3 })
+            .Should()
+            .Be("1,2,3");
+        string.Join(',', new System.Collections.Generic.List<string?> { "a", null, "b" })
+            .Should()
+            .Be("a,,b");
+    }
+
+    [Fact]
+    public void Join_CharSeparator_WithIndexAndCount_Test()
+    {
+        // Arrange
+        var values = new string?[] { "a", "b", "c", "d" };
+
+        // Act & assert
+        string.Join(',', values, 1, 2).Should().Be("b,c");
+        string.Join(',', values, 0, 4).Should().Be("a,b,c,d");
+        string.Join(',', values, 2, 0).Should().Be("");
+    }
 }
diff --git a/PolyShim/NetCore10/String.cs b/PolyShim/NetCore10/String.cs
index b12972b..a6bfe94 100644
--- a/PolyShim/NetCore10/String.cs
+++ b/PolyShim/NetCore10/String.cs
@@ -6,6 +6,7 @@
 // ReSharper disable PartialTypeWithSinglePart
 
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Diagnostics.CodeAnalysis;
 
@@ -19,6 +20,14 @@ internal static class MemberPolyfills_NetCore10_String
         // https://learn.microsoft.com/dotnet/api/system.string.isnullorwhitespace
         public static bool IsNullOrWhiteSpace(string? value) =>
             value is null || value.All(char.IsWhiteSpace);
+
+        // https://learn.microsoft.com/dotnet/api/system.string.join#system-string-join(system-string-system-object())
+        public static string Join(string? separator, params object?[] values) =>
+            string.Join(separator, values.Select(v => v?.ToString()).ToArray());
+
+        // https://learn.microsoft.com/dotnet/api/system.string.join#system-string-join-1(system-string-system-collections-generic-ienumerable((-0)))
+        public static string Join<T>(string? separator, IEnumerable<T> values) =>
+            string.Join(separator, values.Select(v => ((object?)v)?.ToString()).ToArray());
     }
 }
 #endif
diff --git a/PolyShim/NetCore20/String.cs b/PolyShim/NetCore20/String.cs
index 22e1b78..febe45c 100644
--- a/PolyShim/NetCore20/String.cs
+++ b/PolyShim/NetCore20/String.cs
@@ -6,7 +6,9 @@
 // ReSharper disable PartialTypeWithSinglePart
 
 using System;
+using System.Collections.Generic;
 using System.Globalization;
+using System.Linq;
 using System.Text;
 using System.Diagnostics.CodeAnalysis;
 
@@ -140,6 +142,25 @@ public string[] Split(
             string? separator,
             StringSplitOptions options = StringSplitOptions.None
         ) => str.Split([separator ?? ""], options);
+
+        // https://learn.microsoft.com/dotnet/api/system.string.join#system-string-join(system-char-system-object())
+        public static string Join(char separator, params object?[] values) =>
+            string.Join(separator.ToString(), values.Select(v => v?.ToString()).ToArray());
+
+        // https://learn.microsoft.com/dotnet/api/system.string.join#system-string-join(system-char-system-string())
+        public static string Join(char separator, params string?[] values) =>
+            string.Join(separator.ToString(), values);
+
+        // https://learn.microsoft.com/dotnet/api/system.string.join#system-string-join-1(system-char-system-collections-generic-ienumerable((-0)))
+        public static string Join<T>(char separator, IEnumerable<T> values) =>
+            string.Join(
+                separator.ToString(),
+                values.Select(v => ((object?)v)?.ToString()).ToArray()
+            );
+
+        // https://learn.microsoft.com/dotnet/api/system.string.join#system-string-join(system-char-system-string()-system-int32-system-int32)
+        public static string Join(char separator, string?[] value, int startIndex, int count) =>
+            string.Join(separator.ToString(), value, startIndex, count);
     }
 }
 #endif
diff --git a/PolyShim/Signatures.md b/PolyShim/Signatures.md
index 0b398ad..11fec0c 100644
--- a/PolyShim/Signatures.md
+++ b/PolyShim/Signatures.md
@@ -1,8 +1,8 @@
 # Signatures
 
-- **Total:** 373
+- **Total:** 379
 - **Types:** 79
-- **Members:** 294
+- **Members:** 300
 
 ___
 
@@ -371,6 +371,12 @@ ___
   - [`bool IsNullOrWhiteSpace(string?)`](https://learn.microsoft.com/dotnet/api/system.string.isnullorwhitespace) <sup><sub>.NET Core 1.0</sub></sup>
   - [`bool StartsWith(char)`](https://learn.microsoft.com/dotnet/api/system.string.startswith#system-string-startswith(system-char)) <sup><sub>.NET Core 2.0</sub></sup>
   - [`int GetHashCode(StringComparison)`](https://learn.microsoft.com/dotnet/api/system.string.gethashcode#system-string-gethashcode(system-stringcomparison)) <sup><sub>.NET Core 2.0</sub></sup>
+  - [`string Join(char, params object?[])`](https://learn.microsoft.com/dotnet/api/system.string.join#system-string-join(system-char-system-object())) <sup><sub>.NET Core 2.0</sub></sup>
+  - [`string Join(char, params string?[])`](https://learn.microsoft.com/dotnet/api/system.string.join#system-string-join(system-char-system-string())) <sup><sub>.NET Core 2.0</sub></sup>
+  - [`string Join(char, string?[], int, int)`](https://learn.microsoft.com/dotnet/api/system.string.join#system-string-join(system-char-system-string()-system-int32-system-int32)) <sup><sub>.NET Core 2.0</sub></sup>
+  - [`string Join(string?, params object?[])`](https://learn.microsoft.com/dotnet/api/system.string.join#system-string-join(system-string-system-object())) <sup><sub>.NET Core 1.0</sub></sup>
+  - [`string Join<T>(char, IEnumerable<T>)`](https://learn.microsoft.com/dotnet/api/system.string.join#system-string-join-1(system-char-system-collections-generic-ienumerable((-0)))) <sup><sub>.NET Core 2.0</sub></sup>
+  - [`string Join<T>(string?, IEnumerable<T>)`](https://learn.microsoft.com/dotnet/api/system.string.join#system-string-join-1(system-string-system-collections-generic-ienumerable((-0)))) <sup><sub>.NET Core 1.0</sub></sup>
   - [`string Replace(string, string?, bool, CultureInfo?)`](https://learn.microsoft.com/dotnet/api/system.string.replace#system-string-replace(system-string-system-string-system-boolean-system-globalization-cultureinfo)) <sup><sub>.NET Core 2.0</sub></sup>
   - [`string Replace(string, string?, StringComparison)`](https://learn.microsoft.com/dotnet/api/system.string.replace#system-string-replace(system-string-system-string-system-stringcomparison)) <sup><sub>.NET Core 2.0</sub></sup>
   - [`string ReplaceLineEndings()`](https://learn.microsoft.com/dotnet/api/system.string.replacelineendings#system-string-replacelineendings) <sup><sub>.NET 6.0</sub></sup>