— The problem of sensor activation in a controlled discrete event system is considered. Sensors are assumed to be costly and can be turned on/off during the operation of the system. The agent activates sensors as needed in order to observe the trajectories of the system and correctly implement the given feedback control law. Different policies of sensor activation can be used by the agent. A policy is said to be minimal if any strictly less activation prevents the correct implementation of the control law. A systematic formulation of the sensor activation problem is obtained and a solution procedure is proposed. An algorithm that computes minimal sensor activation policies is presented. The algorithm is of polynomial complexity in both the number of states and the number of events in the system.