Intersexual disorders, the causes of which are frequently difficult to diagnose, usually result from the disruption of sex determination and gonadal development or from impairment in function of endocrine organs such as gonads and adrenal glands. Thus consideration of etiopatogenesis of intersexuality should be based on an analysis of the genetic control of sex determination, gonadal differentiation and first of all molecular regulation of steroidogenic cell differentiation and functioning. These processes take place in the fetal gonads, which explains the small amount of data concerning the development of human gonads. However, numerous investigations of gene expression in murine gonads have revealed a complicated machinery involved in sex determination and gonadal differentiation. Moreover, data obtained from mice have led to a better understanding of the molecular background of sex determination and gonad differentiation in humans. Nevertheless, the interpretation of mouse gene knock-outs does not always reflect mutations of their homologues in man. This article compares data in humans and mice, revealing the existence of many sex-determining genes in both, which should be taken into consideration during the diagnosis of intersexual disorders. Mutations of some genes controlling murine gonad development have not been described in humans so far. This indicates the necessity of conducting extensive genetic analysis of individuals with intersexual disorders.