This article describes the control of water chemistry in the steam cycle of a power plant for achieving corrosion control, deposition prevention, and higher cycle efficiency. It discusses the materials requirements of the components exposed to supercritical water in supercritical (SC) and ultrasupercritical (USC) power plants. These components include high-pressure steam piping and headers, superheater and reheater tubing, water wall tubing in the boiler, high-and intermediate-pressure rotors, rotating blades, and bolts in the turbine section. The article reviews the boiler alloys, used in SC and USC boilers, such as ferritic steels, austenitic steels, and nickel-base alloys. It provides information on the materials used in turbine applications such as ferritic rotor steels, turbine blade alloys, and bolting materials. The article explains various factors influencing steamside corrosion in SC power plants. It also deals with the role of overall efficiency in the USC power generation.