Formation of the nitrided case begins through a series of nucleated growth areas on the steel surface. These nucleating growth areas will eventually become what is known as the compound layer or, more commonly, the white layer. This chapter discusses the influence of carbon on the compound zone. It explains how to control and calculate compound zone thickness. Compound zone thickness can be controlled by dilution, the two-stage Floe process, or by ion nitriding. The chapter describes the factors affecting surface case formation.