||Continuous miniaturization of electronics and sensing elements stimulate the evolution of novel unobtrusively integrated smart garments that sense their environment and provide personalized assistance to its wearer. The development of smart garments requires robust integration techniques for electronics and textiles in one common system. Furthermore, recognition algorithms are needed to derive information on the wearer's activity and context within the smart garment. In this work both challenges are addressed in a smart shirt system, called SMASH. SMASH was developed as a rapid prototyping system for smart garment developments. We introduced in this work our approach for prototyping smart garments and present design, implementation, and evaluation of SMASH. The SMASH system embeds a distributed hierarchical architecture of sensing and processing functions in an off-the-shelf long-sleeve shirt. The system design focused on scalability regarding sensors and processing resources, as well as rapid deployment in different applications. We demonstrated the versatility of SMASH in three application evaluations that represent different prototyping phases of smart garments. For these studies several systems of different sizes were implemented. The SMASH system helps to bypass time- and cost-intensive implementation iterations using multiple garment prototypes.