The gas flow in and out of the cylinder is controlled by self actuated, that is gas pressure controlled in- and outlet valves (suction and discharge valves).
In small and single acting compressors with relatively high revolutions per minute preferentially reed valves are used. In medium and large piston compressors nearly always ring or plate valves find application, in special cases also poppet valves. 
With ring valves (1) the valve seat (1) is a plate with slots of concentric circular shape, which are connected by bridges. The free ring areas in the valve seat are sealed by individual non metallic rings (3), which are held against the seat by coil springs (4,5), which in turn are enclosed on the circumference by spring pockets. After building up of a sufficiently high pressure difference over the valve ring the valve opens in a relatively short period of time. The valve lift is limited by a valve guard (2 ) , which again is fitted with slots similar to the seat. Seat and guard are bolted together via a central bolt (6). With plate valves , the seat ports are closed by a common valve plate made from steel or from non metallic material, which again is fitted with slots.
Suction and discharge valves are of the same basic design and only differ with the location of seat and guard in relation to the working chamber.
For proper functioning of the compressor valves the installation in the cylinder has to be adequately designed.
Installion a: Suction and discharge valve are located next to each other in the cylinder cover. The additional clearance volume remains thereby small. The available valve area is, however, very limited. The holding down of the valves is carried out via valve cages and bolts, which are fitted in the valve covers. These have to be sufficiently strong. This design enables easy access to the valves for servicing.
Installation b: By arranging suction and discharge valve concentrically, the valve cover area can be used in full. In order to avoid excessive heat transfer from the warm gas on the delivery side to the gas on the suction side, both valve chambers are separated by a cooled compartment.
Installation a) and b) are in the main limited to simple plunger type compressors.
Installation c: With crosshead compressors, valves are preferentially fitted on the circumference of the cylinder. Thereby valves with larger passage areas can be fitted, which results in smaller pressure drops but also entails larger clearance volumes.
With reed valves the closing of the seat area is done via a self sprung or spring loaded strip of spring steel, the reed . Thereby the design is kept simple and moving masses are low. This design is especially suited for fast running compressors.
When using reed valves, the cylinder head has to be adapted to the reed valve design and has to incorporate large valve chambers (6,7). The common cylinder block (5) of a two cylinder compressor is fitted with a sufficiently stiff common seat plate (1). The suction reeds (2) open into the working chamber and are limited in lift by a stop face on the cylinder circumference. The discharge reeds (3) have a guard (4).