An actuator includes a piezoelectric stack within a housing that applies a compressive load to the stack. The housing may include a cylindrical tube closed at one end and may seal the stack against fluid pressure. The housing may be shorter than the length of the piezoelectric stack and may include resilient means for inducing elastic strain in the stack. The resilient means may include one or more slots extending around the circumference of the housing and/or a corrugated region of the housing to form a tension spring at a lower end of the housing. The tension spring may then serve to draw the stack into compression. The stack may include a plurality of layers of piezoelectric or piezoceramic material interspersed with a plurality of layers of electrically conductive material and connecting means for connecting the layers to a source of electrical power.