diff --git a/IMPLEMENTATION.md b/IMPLEMENTATION.md
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@@ -1 +1,15 @@
-# WIP
+# Implementation Details (WIP)
+
+## WebExtension
+### Permissions
+|Permission|Usage|
+|--|--|
+|`browser.history`|When opening receiver selection popup windows, the history log is polluted unless these entries are removed.|
+|`menus`|Display context menus for casting on pages/media, and whitelist menus on the toolbar button.|
+|`nativeMessaging`|Allows communciation with the bridge.|
+|`notifications`|Show notifications if a bridge issue is found on startup.|
+|`storage`|Store options data.|
+|`tabs`|Execute scripts within a sender application's tab content script context(possibly unnecessary due to host permissions).|
+|`webNavigation`|Get URL of frame to determine available cast media types.|
+|`webRequest`|Intercept and redirect Cast SDK requests.|
+|`<all_urls>`|Wildcard host permission (may want to switch to optional host permissions).|