A large class of distributed applications follow an event-driven or reactive paradigm. Such applications can benefit from Mobile Agent technology by making it easy to add re-configurablity, extensibility, and failure resilience features at an application level. We present the design of a Middleware for building reactive, extensible, reconfigurable distributed systems, based upon an abstraction we call Mobile Streams. Using our system, a distributed, event-driven application can be scripted from a single point of control and dynamically extended and re-configured while it is in execution. Our system is suitable for building a wide variety of applications; for example, distributed test, conferencing and control-oriented applications. We illustrate the use of our system by presenting example applications.