This work deals with the problem of efficient location of sensors and actuators encountered in the domain of active control of flexible structure. It appears that the optimal solution depends upon the type ofcontrol scheme that is used as well as the kinds of sensors and actuators that are implemented, and on the criterion that is considered. This paper recalls and discusses some approaches that are presented in the literature and presents some results that are obtained with a mock-up equipped with piezoelectric sensors and actuators.