diff --git a/CHANGELOG.md b/CHANGELOG.md
index a08b067e..69cbd832 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,12 @@ All notable changes to this project will be documented in this file.
 
 ## [Unreleased]
 
+### Added
+
+* New `EPUBNavigatorDelegate` APIs to inject custom JavaScript.
+  * Override `navigator(_:setupUserScripts:)` to register additional user script to the `WKUserContentController` of each web view.
+  * Override `navigator(_:userContentController:didReceive:)` to receive callbacks from your scripts.
+
 ### Changed
 
 * CocoaPods is not supported anymore.
diff --git a/r2-navigator-swift/EPUB/EPUBNavigatorViewController.swift b/r2-navigator-swift/EPUB/EPUBNavigatorViewController.swift
index 8343da65..7801ecaa 100644
--- a/r2-navigator-swift/EPUB/EPUBNavigatorViewController.swift
+++ b/r2-navigator-swift/EPUB/EPUBNavigatorViewController.swift
@@ -17,7 +17,12 @@ import SwiftSoup
 
 
 public protocol EPUBNavigatorDelegate: VisualNavigatorDelegate {
-    
+
+    // MARK: - WebView Customization
+
+    func navigator(_ navigator: EPUBNavigatorViewController, setupUserScripts userContentController: WKUserContentController)
+    func navigator(_ navigator: EPUBNavigatorViewController, userContentController: WKUserContentController, didReceive message: WKScriptMessage)
+
     // MARK: - Deprecated
     
     // Implement `NavigatorDelegate.navigator(didTapAt:)` instead.
@@ -34,7 +39,10 @@ public protocol EPUBNavigatorDelegate: VisualNavigatorDelegate {
 }
 
 public extension EPUBNavigatorDelegate {
-    
+
+    func navigator(_ navigator: EPUBNavigatorViewController, setupUserScripts userContentController: WKUserContentController) {}
+    func navigator(_ navigator: EPUBNavigatorViewController, userContentController: WKUserContentController, didReceive message: WKScriptMessage) {}
+
     func middleTapHandler() {}
     func willExitPublication(documentIndex: Int, progression: Double?) {}
     func didChangedDocumentPage(currentDocumentIndex: Int) {}
@@ -635,6 +643,10 @@ extension EPUBNavigatorViewController: EPUBSpreadViewDelegate {
         present(viewController, animated: true)
     }
 
+    func spreadView(_ spreadView: EPUBSpreadView, userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
+        delegate?.navigator(self, userContentController: userContentController, didReceive: message)
+    }
+
 }
 
 extension EPUBNavigatorViewController: EditingActionsControllerDelegate {
@@ -663,6 +675,10 @@ extension EPUBNavigatorViewController: PaginationViewDelegate {
             contentInset: config.contentInset
         )
         spreadView.delegate = self
+
+        let userContentController = spreadView.webView.configuration.userContentController
+        delegate?.navigator(self, setupUserScripts: userContentController)
+
         return spreadView
     }
     
diff --git a/r2-navigator-swift/EPUB/EPUBSpreadView.swift b/r2-navigator-swift/EPUB/EPUBSpreadView.swift
index 2eb2e25a..9c588d1a 100644
--- a/r2-navigator-swift/EPUB/EPUBSpreadView.swift
+++ b/r2-navigator-swift/EPUB/EPUBSpreadView.swift
@@ -30,6 +30,9 @@ protocol EPUBSpreadViewDelegate: class {
     
     /// Called when the spread view needs to present a view controller.
     func spreadView(_ spreadView: EPUBSpreadView, present viewController: UIViewController)
+
+    /// Called when the spread view receives an unknown JavaScript message.
+    func spreadView(_ spreadView: EPUBSpreadView, userContentController: WKUserContentController, didReceive message: WKScriptMessage)
     
 }
 
@@ -395,10 +398,11 @@ extension EPUBSpreadView: WKScriptMessageHandler {
 
     /// Handles incoming calls from JS.
     func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
-        guard let handler = JSMessages[message.name] else {
-            return
+        if let handler = JSMessages[message.name]  {
+            handler(message.body)
+        } else {
+            delegate?.spreadView(self, userContentController: userContentController, didReceive: message)
         }
-        handler(message.body)
     }
 
 }