The biomechanics of glenohumeral stability involve several static and dynamic mechanisms to achieve the intricate balance between shoulder mobility and stability. In conjunction with recent in vitro studies, two important stabilizing mechanisms, concavity compression and scapulohumeral balance, were described. Concavity compression refers to the stability obtained by compressing the humeral head into the concave glenoid fossa. Increasing the magnitude of the compressive load, as provided by dynamic muscle contraction, and the depth of the glenoid concavity, which varies from the asymmetric geometry, enhance concavity compression stabilization. The related scapulohumeral balance refers to the dynamic positioning of the glenohumeral joint so that the joint reaction force is balanced within the glenoid fossa. The greater the arc provided by the glenoid, the larger the range of joint force angles acting through the humeral head that may be stabilized. The presence of an intact glenoid labrum is important to both mechanisms. Concavity compression and scapulohumeral balance may be of particular importance to glenohumeral joint stability in the midrange of motion where the capsuloligamentous constraints are lax. Clinical correlation of these mechanisms contributes to the understanding of glenohumeral instability.