Undescended testis (UDT) occurs when something goes wrong with testicular descent from high in the abdominal cavity to the scrotum. Normal descent occurs in two steps, with the transabdominal phase controlled by a new testicular hormone, insulin-like hormone 3, and the inguinoscrotal phase controlled by androgens. The latter phase requires a complex process of migration from the inguinal abdominal wall to the scrotum and is commonly defective, leading to the high incidence (2–4%) of UDT at birth. The clinical examination of babies and infants aims to confirm the persistence of congenital UDT by 3–6 months, so surgery can be optimally timed at 6–12 months. For those boys who develop acquired UDT later in childhood, the ‘ascending’ testis often needs surgery between 5 years and 10 years, so all boys should be screened again for UDT at school entry.