A valve is a device attached to a pipe or tube that regulates the flow of liquid or air through the pipe or tube. Valves are also known as regulators. They are a common component in numerous scenarios; they come in a wide range of sizes and are designed in different ways depending on how they are being used. Valves can also range in complexity from the quite simple to the extremely complicated.

Valves can be used for different purposes, such as managing water flow for irrigation, industrial process control, and residential on/off and pressure control. Valves are found in virtually every industrial process, including water and sewage processing, mining, power generation, processing of oil, gas and petroleum, food manufacturing, chemical and plastic manufacturing and many other fields.

Valves can be manually opened and closed using a handle, lever, pedal, or wheel. Valves may also be automated, responding to changes in flow, temperature, or pressure. This sort of valve is frequently seen in hot water systems or boilers, where safety valves are installed. These changes may act on a diaphragm or a piston, which in turn activates the valve.