Shoulder impingement can be diagnosed based on clinical findings, but clinical staging of its severity, which may range from reversible edema and hemorrhage to a complete rotator cuff tear, is often not possible. MR imaging allows direct examination of the thickness, signal characteristics, and continuity of the rotator cuff. Furthermore, MR imaging can be used not only to determine the status of the cuff, but also to detect pathology involving surrounding structures and to identify anatomic variations that may contribute to the impingement. Rapid MR imaging during active unguided shoulder motion will likely further increase our understanding of the biomechanics of the shoulder at different stages of impingement, and may shed further light on the pathogenesis of this disorder.