The shoulder is an inherently unstable joint that is subject to different patterns of instability. Determining the direction of subluxation that is causing the patients symptoms can be difficult. Although posterior and multidirectional instability share many characteristics, they have different etiologies and treatment requirements. Multidirectional instability was first described in 1980 by Neer and Foster, but the continuing lack of a consistent definition for the condition contributes to difficulty in both diagnosis and treatment. Posterior instability has been more precisely defined, but the diagnosis nonetheless can be difficult. For most patients with either condition, rehabilitation and bracing are the preferred treatment. If nonsurgical treatment is unsuccessful, arthroscopic treatment can provide a satisfactory result.