The development of piezoelectric actuators for active e ow control is discussed. The type of actuators considered consists of a single sheet of piezoceramic material bonded to the underside of a shim: a “ unimorph” e ap design. Existing theoretical beam models are extended to incorporate a linear strain distribution in the composite unimorph beam structure. This model is combined with an optimization scheme to design a e ap that maximizes the tip dee ection per unit voltage for a given bandwidth. The optimization model is then used to design a piezoelectric actuator. The model predictions compare favorably to measurements of the actuator frequency response function. A sample application to control of separated e ow from a backward-facing step is also described, in which the actuator is installed at the origin of the free shear layer. Detailed hot-wire measurements, together with dimensional analysis, reveal the physical mechanism responsible for the e uid-structure coupling. A quasi-static model based on the solid-body displacement of the incoming shear layer accurately describes the peak streamwise velocity perturbations produced by the actuator. This model leads to a proportional relationship between the e ap tip displacement, the incoming boundary-layer proe le parameters, and the streamwise velocity e uctuations produced by the actuator.