Overhead athletes require a delicate balance of shoulder mobility and stability in order to meet the functional demands of their respective sport. Altered shoulder mobility has been reported in overhead athletes and is thought to develop secondary to adaptive structural changes to the joint resulting from the extreme physiological demands of overhead activity. Researchers have speculated as to whether these structural adaptations compromise shoulder stability, thus exposing the overhead athlete to shoulder injury. Debate continues as to whether these altered mobility patterns arise from soft-tissue or osseous adaptations within and around the shoulder. Researchers have used quantitative techniques in an attempt to better characterize these structural adaptations in the shoulders of overhead athletes. Throwing athletes have been shown to display altered rotational range of motion (ROM) patterns in the dominant shoulder that favour increased external rotation and limited internal rotation ROM. Throwers also show a loss of horizontal or cross-body adduction in the throwing shoulder when compared with the non-throwing shoulder. This posterior shoulder immobility in the throwing shoulder is thought by some researchers to be associated with reactive scarring or contracture of the periscapular soft-tissue structures (e.g. posterior capsule and/or cuff musculature); however, evidence of reactive scarring or contractures of the posterior-inferior capsule or cuff musculature from anatomic or noninvasive imaging studies is lacking. Conversely, translational ROM (laxity) has been consistently shown to be symmetric between dominant and non-dominant shoulders of overhead athletes. From a skeletal perspective, throwing shoulders are shown to have more humeral retroversion when compared with the non-throwing shoulder. Alterations in humeral retroversion are thought to develop over time in young pre-adolescent throwers when the proximal humeral epiphysis is not yet completely fused. Even though the evidence is inconclusive at the present time, there is more compelling evidence that leads us to believe that altered shoulder mobility in the overhead-throwing athlete is more strongly associated with adaptive changes in proximal humeral anatomy (i.e. retroversion) than to structural changes in the articular and periarticular soft tissue structures. In addition, this retroversion is thought to account for the observed shift in the arc of rotational ROM in overhead athletes. However, in some athletes, capsulo-ligamentous adaptations such as anterior-inferior stretching or posterior-inferior contracture may become superimposed upon the osseous changes. This may ultimately lead to pathological manifestations such as secondary impingement, type II superior labrum from anterior to posterior (SLAP) lesions and/or internal (glenoid) impingement. Overuse injuries in the overhead athlete are a common and perplexing clinical problem in sports medicine and, therefore, it is imperative for sports medicine clinicians to have a thorough understanding of the short- and long-term effects of overhead activity on the shoulder complex. It is our intention that the information presented will serve as a guide for clinicians who treat the shoulders of overhead athletes.