The transmission of digital video over wireless networks is becoming a reality: it is now possible to construct working prototype systems which illustrate the benefits to be accrued from the integration of mobile computing and digital video. However, systems which deploy video in mobile environments must be able to adapt to changes in the quality of service of their underlying communications channel. The authors focus on the practical applications and implications of supporting adaptive video in mobile environments. In particular, we describe a testbed which supports multicast transmission of stored and live video sequences over both WaveLAN and GSM technologies. The testbed employs H.263 and MPEG encoding techniques and enables clients to freely roam between heterogeneous networks while maintaining video connectivity.