The steam is admitted into the cylinder of an engine by means of valves, as previously stated. There are three distinct types of valve - (1) slide, (2) Corliss, and (3) poppet valves.

The slide valve is a simple casting similar in its lengthwise section to the letter D. By being moved back and forth over the steam ports of the engine it admits and exhausts the steam alternately, thus causing the piston in the cylinder to work back and forth.

Corliss valves are semirotary valves, cylindrical in shape, which partly turn in cylindrical chambers.

Poppet valves are simply disks attached to a stem, which work over a circular opening. They are raised and lowered over the parts.

The mechanism controlling these valves is called the valve gear.