diff --git a/Sources/Then/Then.swift b/Sources/Then/Then.swift
index 3b91b74..741d265 100644
--- a/Sources/Then/Then.swift
+++ b/Sources/Then/Then.swift
@@ -79,6 +79,7 @@ extension CGSize: Then {}
 extension CGVector: Then {}
 extension Array: Then {}
 extension Dictionary: Then {}
+extension Set: Then {}
 
 #if os(iOS) || os(tvOS)
   extension UIEdgeInsets: Then {}
diff --git a/Tests/ThenTests/ThenTests.swift b/Tests/ThenTests/ThenTests.swift
index eefa31a..4cecef1 100644
--- a/Tests/ThenTests/ThenTests.swift
+++ b/Tests/ThenTests/ThenTests.swift
@@ -47,6 +47,13 @@ class ThenTests: XCTestCase {
     XCTAssertEqual(dict, ["Korea": "Seoul", "Japan": "Tokyo", "China": "Beijing"])
   }
 
+  func testWith_Set() {
+    let set = Set(["A", "B", "C"]).with {
+      $0.insert("D")
+    }
+    XCTAssertEqual(set, Set(["A", "B", "C", "D"]))
+  }
+
   func testDo() {
     UserDefaults.standard.do {
       $0.removeObject(forKey: "username")