Hypospadias is one of the most common congenital defects in humans. The molecular events required in the genitourinary tract for normal development of the external genitalia are just beginning to be elucidated. Identifying the cause of hypospadias may be relevant not only in the field of pediatric urology, but also for worldwide public health. This article discusses the possible causes of hypospadias and the genes and growth factors involved. The potential effects of genetic susceptibility and environmental pollutants are also discussed.