Abstract: A method and apparatus for controlling a compressor is disclosed wherein both the compressed air output and the speed of the compressor are simultaneously adjusted in relation to compressed air demand over a wide range of air demands and in a way so as to use less input energy during below-peak demand and automatically return the compressor to an operating condition of minimum speed and minimum power input when there is no demand on the compressor. A reciprocating piston compressor with at least two, and typically more than two, compression chambers driven by a prime mover has a separate unloader device for each compression chamber. A control assembly is responsive to tank pressures indicating demand on the compressor with the movement of one movable control member simultaneously operating both a control valve adapted for successively operating each of the unloader devices one at a time and a speed regulating member for the prime mover.