Glenohumeral internal rotation deficit (GIRD) in baseball pitchers commonly develops before puberty. GIRD is also a potential factor leading to subsequent shoulder instability or shoulder impingement. The etiology of GIRD can be explained in osseous adaptation and soft tissue adaptation, including humeral retroversion and posterior shoulder tightness. GIRD could also affect pitching control as a result of insufficient shoulder horizontal adduction and the scapular dyskinesis. GIRD could be improved through physical therapy. Early intervention with appropriate stretching programme may reduce the development of GIRD and subsequent shoulder instability.