A robot must often react to events in its environment and exceptional conditions by suspending or abandoning its current plan and selecting a new plan that is an appropriate response to the event. This paper describes how high-level controllers for robots that are reactive in this sense can conveniently be implemented in ConGolog, a new logic-based robot/agent programming language. Reactivity is achieved by exploiting ConGolog’s prioritized concurrent processes and interrupts facilities. The language also provides nondeterministic constructs that support a form of planning. Program execution relies on a declarative domain theory to model the state of the robot and its environment. The approach is illustrated with a mail delivery application.