— Android: NDK, Material Design, AppCompat, Fragments, Content Providers, Services, Location and Sensors, Camera, Ui Tests, AdMob; — Java Core: Java Core, Multithreading, Collections API, etc; — IDE: Android Studio, IntelliJ Idea, Eclipse; — API: Firebase, Google Maps, Google Places, Instagram, Facebook, Vkontakte, Twitter, — Security: OpenVPN, IKEv2 (strongSwan) — VCS: Git, Mercurial — Bug/Issue trackers: Jira, YouTrack; — Other: JSON, XML, REST, SQL, SOAP