In this thesis, we contribute to the body of surface computing research by proposing novel methods for user identification. We present three novel methods for instantaneous user identification: IdWristbands uses bracelets that emit infrared codes to identify individual finger touches, HandsDown is based on biometrics and allows users to identify by placing their hand flat on the surface, and PhoneTouch employs mobile phones in a stylus-like fashion for identified direct-touch interaction. We use IdWristbands, HandsDown, and PhoneTouch as basis for analyzing and exploring the design space of personalized interaction. We illustrate its opportunities and benefits by introducing a wide range of novel user-aware interaction techniques. In a user study, we show that instantaneous user identification and personalization conveniently facilitate interaction techniques that are otherwise not immediately accessible or difficult to realize.