Os acromiale is an uncommon cause of shoulder pain with symptoms often resulting from an unstable meso-acromion. The associated pain may be due to impingement from the unfused fragment, a concomitant rotator cuff tear, or gross motion at the os acromiale site. Currently, initial treatment includes physical therapy, nonsteroidal anti-inflammatory agents, and subacromial corticosteroid injections. Surgical intervention is reserved for patients who do not respond to nonoperative treatment. Treatment options include open fragment excision, open reduction and internal fixation, and arthroscopic decompression. Open fragment excision can lead to persistent deltoid dysfunction and should be reserved for small fragments or after failed internal fixation. Open reduction and internal fixation allows for both preservation of large fragments and anterior deltoid function. Internal fixation is technically difficult, has led to frequent nonunion rates and often requires hardware removal as a result of postoperative irritation. Arthroscopic subacromial decompression with complete or nearly complete resection of the unstable meso-acromion can be performed without the aforementioned complications. The surgical technique requires no special instrumentation and may be performed reproducibly by those familiar with arthroscopic techniques of the shoulder. Advantages include more rapid rehabilitation, better range of motion, and shorter surgical times. Satisfactory short-term results have shown this to be an effective treatment option for the unstable meso-acromion.