Abstract: A rotary shear seal hydraulic valve which can be used in three- and four-way embodiments is formed by a housing and a body together defining therebetween a generally cylindrical stepped cavity in which a rotor is rotatably positioned. The housing contains at least two shear seal valve assemblies positioned to be aligned with ports in the rotor to establish communication between the shear seal means. The body contains at least one piston member including a rack portion engaging a profiled pinion portion of the rotor to effect the rotational drive of the rotor. The pistons can be driven in both directions by fluid means or in one direction by spring return means. The pistons can also be actuated by solenoid means and the positioning of the rotor can be strained by a lever providing a manual override. By selectively plugging the various ports, the four-way valve can be utilized in a three-way configuration, thereby providing a great degree of flexibility.