Many distributed applications can be cast as reactive systems, where a reactive system consists of an instrumented program that is monitored and controlled by an input-driven control program. Examples of non-real-time reactive systems include monitoring and debugging systems, tool integration services, and network and distributed application managers. There is currently little support for building reactive systems. This paper describes the Meta toolkit that provides such support. Using Meta, a distributed system can be instrumented with a sensor and actuator abstraction that exposes the state of the system for purposes of control. Then, a control program can be written that interacts with the instrumented system using guarded commands. Of particular concern is the efficiency of control, so Meta allows the control program to be distributed in order to take advantage of locality as much as possible.