This text reviews the types, design and usage of control valves in the process industries. It also discusses factors such as sizing, materials construction, the type of chemical flowing through the valve and maintenance. Technologies that affect the usage of valves are also considered. A control valve is a valve which regulates the flow or pressure of a medium which affects some controlled process. Control valves are usually operated by some remote signals by independent devices using control mechanisms powered electronically, pneumatically or electro-hydraulically. The text demonstrates how control valves are integrated within the control loop and explains recently develop diagnostic techniques to help the engineer isolate and solve problems in particular elements. Guidelines based on valve dynamics are provided as a measure for loop tuning and maintenance for optimal performance.