Instability of the distal radioulnar joint is a source of ulnar-sided wrist pain and functional impairment. Symptomatic instability may present acutely, after a recent traumatic injury, or in a delayed fashion as chronic instability following a history of a traumatic event. A detailed understanding of the complex anatomy, biomechanics, and stabilizing structures of the distal radioulnar joint is important to evaluate and treat acute and chronic instability. This article describes the pathophysiology of distal radioulnar joint instability with attention to the anatomic structures involved and provides information about evaluation and treatment.