This chapter reviews some recent advances in our understanding of human sex development, and discusses a newly suggested nomenclature and classification system for disorders of sex development (DSD). It provides a brief overview of some known genetic causes of DSD, and outlines some challenges for research and clinical practice in this field related to the theme of reproductive genetics. Sex chromosome DSD includes all cases of sex chromosome aneuploidy. Complete gonadal dysgenesis results in a complete lack of androgenisation and a female phenotype. The most common cause of androgen excess is congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency. Increasing number of cases of DSD are being diagnosed earlier owing to discordance between a karyotype performed in the first trimester and genital appearance on prenatal ultrasound or at birth. The early postnatal management of a child born with ambiguous genitalia or karyotype/phenotype discordance is very important.