Injury to the ulnar collateral ligament (UCL) of the thumb metacarpophalangeal (MCP) joint is a common entity encountered by the sports physician and orthopedic surgeon. The term "gamekeeper's thumb," which is sometimes used incorrectly to mean any injury to this ligament, refers to a chronic injury to the UCL in which it becomes attenuated through repetitive stress. In contrast, the term "skier's thumb" refers to an acute ligament injury as seen in skiers who fall on an abducted thumb or athletes who sustain a valgus force on an abducted thumb. If the patient allows a clinical examination, valgus stress testing can diagnose a complete UCL rupture when there is no solid endpoint with the thumb held in 30 degrees of MCP flexion and with the thumb held in extension. In cases with complete UCL tears, operative treatment has been shown to produce excellent results and is recommended. If there is a firm endpoint to valgus stress testing, a partial UCL tear is diagnosed and nonoperative treatment usually favored.