Laptop computers and digital technologies are more and more being used in live music performance. However a central issue regarding the aesthetics of laptop performance, compared to traditional instruments, is the absence of physicality. This has implications both for performer and for audience. In order to lend more gestural control to the performer, and appreciation thereof to the audience, this thesis presents a new musical interface. "Laptap" is a laptop-based, real-time sound synthesis/control system for music and multimedia performance. The system produces unique sounds by positive acoustic feedback between the on-board microphone and speaker of a laptop computer. Users can make a variety of sounds by touching the laptop computer in several different ways, and control timbre with gestures of the hands above the microphone and the speakers to manipulate the characteristics of the acoustic feedback path. This thesis introduces the conceptual foundations of the instrument, describes its features for sound generation and manipulation, and reports the results of public performance and user evaluation. Finally, some relevant research topics that might follow in the future are suggested.