Purpose of Review <p>The main components of the shoulder complex, including the glenohumeral and scapulothoracic articulations, are both dependent on a balance of dynamic force couples to facilitate motion and function. The purpose of this review is to summarize when a disruption of the scapular force couple has a direct impact on the glenohumeral force couple and associated stability; how to diagnose this and ultimately treat it.</p> Recent Findings <p>The coordinated motion between the two articulations, termed scapulohumeral rhythm (SHR), drives the shoulder to do many types of motions and function in many directions. The relationship between the glenohumeral and scapulothoracic articulations is not only important for shoulder function, but pathologic processes impacting one also impact the other. This can be seen in excessive scapular compensation in the setting of a rotator cuff tear, or excessive glenohumeral compensation in the setting of a paralyzed serratus anterior. This relationship is particularly important in the setting of recurrent shoulder instability, where scapulothoracic abnormal motion patterns predispose the glenohumeral joint to subluxate or dislocate. When the scapula either loses its ability to externally rotate, remains excessively protracted, or functions with abnormal uncoordinated motion patterns, its uncoordinated movement and resting position predisposes the shoulder joint to be unstable, particularly in the posterior direction. When addressing shoulder instability, it is critical to consider these motion patterns and address them when present, or the patient will be predisposed to continuing to have recurrent shoulder instability.</p> Summary <p> In this review, we will highlight the critical role of glenohumeral and scapulothoracic dynamic force couples and muscle balances, as well as the important impact scapulothoracic pathology can have on glenohumeral joint stability.</p>