diff --git a/.swiftlint.yml b/.swiftlint.yml
index 8d99afc..31908d3 100755
--- a/.swiftlint.yml
+++ b/.swiftlint.yml
@@ -38,6 +38,12 @@ file_header:
                     \/\/
 
 custom_rules:
+  class_semantics:
+    included: ".*.swift"
+    regex: '^(?!.*(final|abstract|recursive)).*class [A-Z]+.*\{'
+    name: "Class Semantics Specification"
+    message: "Specify the class semantics with either final, /* abstract */ or /* recursive */ keywords. Recursive means that class is used both itself & as a superclass."
+    severity: warning
   closing_brace_whitespace:
     included: ".*.swift"
     regex: '(?:\n| {2,})\}\)? *\n *[^ \n\})\]]'
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 1052b02..918b885 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -33,6 +33,7 @@ Please obey these guidelines:
     - Write expressive tests covering any edited / newly created code.
     - Within the `Collections`, `Helpers` and `Tests/Sources` folders, sort things alphabetically.
 - For **file headers** in Swift source files, use the following format:
+
 ```swift
 //
 //  [File Title].swift
diff --git a/Helpers/Weak.swift b/Helpers/Weak.swift
index 5d73026..40eb03d 100644
--- a/Helpers/Weak.swift
+++ b/Helpers/Weak.swift
@@ -9,7 +9,7 @@
 import Foundation
 
 /// Weak wrapper for generic AnyObject.
-class Weak<T>: Hashable where T: AnyObject {
+final class Weak<T>: Hashable where T: AnyObject {
     // MARK: - Properties
     let hashValue: Int
     private(set) weak var value: T?
diff --git a/Tests/Sources/BagTests.swift b/Tests/Sources/BagTests.swift
index 98202cd..d78cc10 100644
--- a/Tests/Sources/BagTests.swift
+++ b/Tests/Sources/BagTests.swift
@@ -9,7 +9,7 @@
 @testable import SwiftCollections
 import XCTest
 
-class BagTests: XCTestCase {
+final class BagTests: XCTestCase {
     // MARK: - Properties
     private lazy var baseBag: Bag<Int> = {
         // Assuming array init works properly
diff --git a/Tests/Sources/WeakArrayTests.swift b/Tests/Sources/WeakArrayTests.swift
index cc9154c..b367569 100644
--- a/Tests/Sources/WeakArrayTests.swift
+++ b/Tests/Sources/WeakArrayTests.swift
@@ -9,9 +9,9 @@
 @testable import SwiftCollections
 import XCTest
 
-class WeakArrayTests: XCTestCase {
+final class WeakArrayTests: XCTestCase {
     // MARK: - Subtypes
-    class SampleObject { }
+    final class SampleObject { }
 
     // MARK: - Properties
     private let arraySize = 3
diff --git a/Tests/Sources/WeakDictionaryTests.swift b/Tests/Sources/WeakDictionaryTests.swift
index af29213..6875b2f 100644
--- a/Tests/Sources/WeakDictionaryTests.swift
+++ b/Tests/Sources/WeakDictionaryTests.swift
@@ -11,9 +11,9 @@ import Foundation
 @testable import SwiftCollections
 import XCTest
 
-class WeakDictionaryTests: XCTestCase {
+final class WeakDictionaryTests: XCTestCase {
     // MARK: - Subtypes
-    class SampleObject: Hashable {
+    final class SampleObject: Hashable {
         var hashValue: Int
 
         init(_ hashValue: Int) {
diff --git a/Tests/Sources/WeakSetTests.swift b/Tests/Sources/WeakSetTests.swift
index e3414ba..efbddd9 100644
--- a/Tests/Sources/WeakSetTests.swift
+++ b/Tests/Sources/WeakSetTests.swift
@@ -9,9 +9,9 @@
 @testable import SwiftCollections
 import XCTest
 
-class WeakSetTests: XCTestCase {
+final class WeakSetTests: XCTestCase {
     // MARK: - Subtypes
-    class SampleObject: Hashable {
+    final class SampleObject: Hashable {
         var hashValue: Int
 
         init(_ hashValue: Int) {