A large class of distributed testing, control and collaborative applications are reactive or event driven in nature. Such applications can be structured as a set of handlers that react to events and that in turn can trigger other events. We have developed an application building toolkit that facilitates development of such applications. Our system is based on the concept of Mobile Streams. Applications developed in our system are dynamically extensible and re-configurable and our system provides the application designer a means to control how the system can be extended and reconfigured. We describe our system model and implementation and compare our design to the design of other systems. (Also cross-referenced as UMIACS-TR-98-36)