Posterior glenohumeral instability (PGHI) is a commonly under- and misdiagnosed pathology owing to its variety of clinical presentations. In order to facilitate diagnosis and treatment, the simple yet comprehensive ABC classification for PGHI is based on the underlying pathomechanical principles and current standard of treatment. Three main groups of PGHI are distinguished based on the type of instability: A (first time), B (dynamic), C (static). Two subtypes further differentiate these groups in terms of their specific pathomechanism and provide a guideline in the choice of appropriate treatment: A (1, subluxation; 2, dislocation), B (1, functional; 2, structural), C (1, constitutional; 2, acquired). While conservative treatment is warranted in most patients with type 1 PGHI (A1, B1, C1), surgical treatment should be considered on an individual basis in patients with type 2 PGHI (A2, B2, C2), while keeping in mind that the different groups and subtypes can overlap, co-exist, or even progress from one to another over time. Of course the necessity for surgical treatment depends on the extent of the structural defects, on the severity of symptoms, on the chronicity, as well as on patient-specific functional demand, age, and health status. Nonetheless, the ABC classification helps to correctly diagnose the type of PGHI and provides a guideline for the generally recommended type of treatment.