The purpose of this chapter is to describe briefly what is known about normal sexual differentiation and to consider the causes, diagnosis, and management of disorders in such differentiation. Abnormalities of embryonic development can be classified in several ways, and the disorders of sexual differentiation are no exception. The categorization presented here differs from that in most textbooks but is well suited to the clinician attempting to consider these disorders in a logical fashion. Disorders of the hypothalamic-pituitary axis sometimes included in such discussions are not considered in this chapter, because they are not truly errors of sexual differentiation and because they are discussed elsewhere in this book (see Chap. 16, Chap. 18, Chap. 92, Chap. 93, Chap. 96 and Chap. 115). Some disorders dealt with briefly here are discussed in detail in other chapters.