A valve is a device which can be used to control the flow of liquids, gases, and slurries. Also known as regulators, valves can be found in almost any situation: chances are that you are sitting within a few feet of a mechanical valve right now. There are a number of different designs for valves, depending on how they are being used, and they can be found in a range of sizes from smaller than a pinky to gigantic. Valves also vary from the extremely basic to the extraordinarily complex, and they are one of the oldest mechanical designs; basic valves have been in use for thousands of years.

The term "valve" can be used to refer to human anatomy as well as a mechanical device. Valves throughout the body regulate the flow of blood, oxygen, and body fluids. Valves in the body include the extremely important heart valve, which works with the heart to pump blood through the body. Individuals with heart disease may have their heart valves replaced with artificial ones to perform this vital body function. The numerous valves in the human body work together to keep things running smoothly in your daily life.

Numerous permutations of the mechanical valve exist, but the following are common valves you probably see and use on a daily basis. The most basic mechanical valve is a gate valve, which has two positions: open, and closed. A globe valve is slightly more complex, controlling not only the flow of liquid, but also the amount, and is also called a throttle. For example, most taps are forms of a globe valve, allowing users to turn the water on and determine how much water is going to flow out. Manufacturers also make check valves, designed to restrict the flow of a substance to one direction only, and safety valves, which can release dangerous levels of pressure.

A valve can be manually controlled, like a tap, or controlled by a large system, as is the case in a car, where valves open and close to permit combustible fuel to enter the cylinder. In the case of a safety valve, the valve may be set to trigger when a certain pressure level is reached, or when an emergency signal is sent out. Usually, a safety valve is equipped with a small spring that will hold it shut until it needs to be opened. In other instances, valves may be controlled by users through a computer system, as is the case with oil, gas, and water pipelines that sprawl for miles: technicians in a central control center can open and close valves remotely, as needed.