Overhand throwing places high loads and stresses on the joints and tissues of the shoulder and arm. As a result, throwing athletes regularly demonstrate altered shoulder internal and external ranges of motion where internal rotation (IR) is decreased and external rotation is increased in the dominant arm when compared with the nondominant arm. This alteration can exist as a result of alterations to the bones (humeral retroversion), capsule (posterior thickening), or muscle (passive stiffness known as thixotropy). When the amount of IR or total arc of motion difference reaches a certain threshold (typically 20 or more degrees of IR or 8 degrees total arc difference), it is known as glenohumeral internal rotation deficit or total arc of motion deficit. Glenohumeral internal rotation deficit and total arc of motion deficit can cause alterations in biomechanics such as scapular “wind-up” or alteration of glenohumeral joint kinematics, which can in turn lead to clinical findings of impingement and labral pathology. This study will review the causes of motion alteration, effects of altered motion on the throwing motion, provide definitions for the various types of rotation deficits, and how to evaluate and treat rotational deficits.