Accurate positioning of the prosthetic humeral head is necessary to reproduce normal glenohumeral kinematics and to avoid damage to the rotator cuff and impingement on the glenoid component or coracoacromial arch. Proper positioning of the head requires accurate placement of the stem and prosthetic designs that allow the head position to adapt to the variations in both normal and pathologic humeral anatomy. Glenoid malpositioning can lead to both humeral instability and increased stress of the glenoid component that may lead to premature glenoid loosening. This review summarizes the cadaveric and finite-element model that defines the abnormalities associated with humeral and glenoid component malpositioning.