Control valves are required in pneumatic and hydraulic systems to direct and regulate the flow of fluid from compressor or pump to the various load devices. Relief valves are infinite position valves. Finite position valves are used for passage or blocking of fluid flow. There can be either two or four parts for control valves. The three major types of control valves are poppet valves, rotary valves and spool valves. Simple discs, balls and cones are used in poppet valves. The two varieties of poppet valves are 2/2 simple valve and 3/2 valve. Poppet valves are economical but require large operating force. Rotary valves are used for hand operation in pneumatic systems. They are compact and have low operating forces. Spool valves are constructed with a spool moving horizontally within the valve body. The spool can be easily moved as there is no net force acting on the spool due to system pressure. There are two types of spool valves. They are two-way spool valve and four-way spool valve. Spools are shifted during the operation of spool valves. For this, a button, lever or a solenoid is used.