A valve stem is a self-contained valve which opens to admit gas to a cavity, and is then automatically closed and kept sealed by the pressure in the cavity, or by a spring to restrict gas (as used in Natural gas) from escaping.

The stem transmits motion from a controlling device to the valve disc. The stem naturally passes through the bonnet when included. Sometimes, the stem and the disc can be combined as one, or the stem and the handle are combined as one. The valve stem seal and a bonnet member are commonly housed within the valve body.

The action transmitted by the stem may be back and forth, rotating, or combination of these actions.

Packing is often used between the stem and the bonnet to maintain a seal. Some valves have no external control and do not need a stem as is the case in nearly all check valves.

Valves that have a disc between the seat and the stem and where the stem moves into the valve to shut it are referred to as being, normally-seated or front seated. Valves that have a seat between the disc and the stem and where the stem moves out of the valve to shut it are referred to as reverse-seated or back seated valves.