A new classification for undescended testis (UDT), suitable for a clinical setting, is proposed. UDT is categorized into congenital and acquired forms. Congenital forms include intra-abdominal, intra-canalicular, supra-scrotal and ectopic testes. Acquired forms can be divided into primary and secondary types. Primary forms are described as either ascending testes, i.e. those which cannot be manipulated into the scrotum, or high scrotal testes, i.e. those which can still be brought through the scrotal entrance into a high scrotal (unstable) position. Secondary forms are the result of ipsilateral groin surgery and are termed "trapped testes". Congenital forms of UDT should be treated surgically at an early age, preferably at 1 year. Therapy for primary acquired forms remains controversial. Therapeutic modalities include orchidopexy, hormonal treatment (preferably with human chorionic gonadotrophin) or waiting for spontaneous descent during the peripubertal period ("laissez faire policy"). Secondary acquired forms are probably best treated surgically.