This review describes recent research into the cause of testicular descent and discusses two controversial theories on the mechanism. New animal models of cryptorchidism are described which shed light on the possible aetiology. A brief summary is given of current management, highlighting the controversies of timing of surgery, use of hormone therapy and congenital versus 'acquired' undescended testes. New information about germ cell development in the postnatal testis is changing the way we view treatment and the ultimate prognosis.