Diffusion is the primary mechanism by which carbon atoms move or migrate in iron. It is driven by concentration gradients and aided by heat. This chapter provides a practical understanding of the diffusion process and its role in the production and treatment of steel. It discusses the factors that determine diffusion rates and distances, including time, temperature, and the relative size of the atoms involved. It also describes two heat treating methods, carburizing and decarburizing, where carbon diffusion plays a central role.