This article describes the thermodynamics and kinetics of gas carburizing reactions, and details the mass transfer mechanism during gas carburizing. It discusses the various considerations involved in carburizing process planning, and reviews successful operation of the gas carburizing process based on the control of three principal variables: temperature, atmosphere composition or carbon potential, and time. The article also describes the selection criteria for alloy, carbon sources, atmosphere types, and carbon monoxide level for endothermic carburizing atmospheres. It provides information on carburizing modeling, case depth prediction, case depth measurement, and case depth evaluation as well as on carburizing equipment, and also covers the factors affecting distortion after carburizing.