Untreated ulnar collateral ligament (UCL) injury of the thumb metacarpophalangeal joint (MCPJ) can lead to long-term pain and functional limitations.Detection of a UCL injury involves sequential examination of true and accessory ligaments and comparison with the uninjured side.Acute partial UCL injuries can be successfully treated nonoperatively.Acute complete or displaced UCL injuries can be successfully treated with operative repair.In cases of chronic UCL injury, treatment options include static and dynamic reconstructions.If painful arthrosis is present with chronic UCL instability, salvage may be performed with MCPJ fusion.