Instability of the ulnar collateral ligament (UCL) is disabling and can occur at any age as an acute or chronic condition. A medical history and objective examination together with the radiological findings are sufficient to make the diagnosis in the majority of cases. Otherwise, an ultrasound or MRI scan should be considered for the acute and chronic injuries, respectively. The partial injuries are treated non-surgically with immobilisation of the metacarpophalangeal joint, while the complete ruptures require surgery, which is summarised in this review. Chronic UCL injuries with symptomatic osteoarthritis are treated with arthrodesis.