Shoulder instabilities have been classified according to the etiology, the direction of instability, or on combinations thereof. The current authors describe a classification system, which distinguishes between static instabilities, dynamic instabilities, and voluntary dislocation. Static instabilities are defined by the absence of classic symptoms of instability and are associated with rotator cuff or degenerative joint disease. The diagnosis is radiologic, not clinical. Dynamic instabilities are initiated by a trauma and may be associated with capsulolabral lesions, defined glenoid rim lesions, or with hyperlaxity. They may be unidirectional or multidirectional. Voluntary dislocation is classified separately because dislocations do not occur inadvertently but under voluntary control of the patient.