Mechanical devices that control the flow of fluids and gases within an engine and other systems. They are crucial for regulating intake and exhaust gases in the combustion process, ensuring proper engine operation. Valves also manage the flow of fluids like coolant and oil, maintaining optimal temperature and lubrication. Different types of valves, such as intake valves, exhaust valves, and pressure relief valves, perform specific functions, contributing to the engine's efficiency, performance, and longevity.