Publisher Summary This chapter defines the control valve sizing formulas, which involve liquid formulas, gas and vapors, and special conditions. Special conditions in this chapter explains that equations are given to correct for flashing, cavitation, high-viscosity fluids, and mixed phase flow, where the simplified equations may lead to inaccurate sizing. These additional expressions, together with the FCI equations, provide satisfactory solutions for most control valve sizing applications. The simplified equation for gases and vapors agrees favorably with experimental data when used for conventional globe valves, but agreement with experimental data is poor when used with high recovery valves. The chapter also explores that a control valve may be thought of as a section of pipe with a variable resistance. Control valves are typically used to match pumps to the pipeline system characteristics.