This paper presents a method for controlling a quarter-car hydraulic active suspension system using SMC techniques. The method guarantees the stability of the existence of a sliding mode and also the stability of the overall system. The proposed method considers many phases. In the first phase, the suspension dynamics is controlled via the actuator between the sprung and unsprung masses. Then the spool valve displacement dynamics is considered to control the current of the servo valve. Since there is an unknown parameter in the system an adaptation law is proposed to yield an appropriate estimate.