Posterior shoulder instability is often hard to diagnose with clinical examination. Patients generally present with vague pain, weakness, and/or joint clicking but less frequently complaining of frank sensation of instability. Imaging examinations, especially MR imaging and magnetic resonance arthrography, have a pivotal role in the identification and management of this condition. This review describes the pathologic micro/macrotraumatic magnetic resonance features of posterior shoulder instability as well as the underlying joint abnormalities predisposing to this condition, including developmental anomalies of the glenoid fossa, humeral head, posterior labrum, and capsular and ligamentous structures.