Mobile Usability Testing . ein toolbasiertes Vorgehensmodell zum Rapid Prototyping und Usability Testing von Mobilen Systemen im originären Benutzungskontext Mobile Usability Testing Rapid Prototyping Usability Testing Mobile Systems Usability Context Framework. Mobile systems are indispensable in our daily life. Therefore effective processes are necessary in order to cope with the increased requirements by the end users and the high cost and time pressure. Whereas usability testing is mostly standardized for stationary systems mobile systems face new challenges according to their heterogeneity. This dissertation seeks to improve the design and development in order to provide intuitive and innovative mobile systems by strengthening the consideration of the users requirements as well as the context of use within early stages of the design process. The core contribution is the development and evaluation of a tool based process model consisting of a phase model and an rapid prototyping and usability testing instrument. It enables a) to develop prototypes and perform usability tests in early product stages b) usability testing within the native context of use and on real devices and c) to collect valuable data. Additional features are remote observation and simulation in terms of the Wizard of Oz method. The empirical studies confirmed the concept of the tool based process model and showed a close connection between rapid prototyping and usability testing.