After placement of a reverse shoulder endoprosthesis, range of motion is usually still compromised. To what extent this occurs from limitation in motion of the reverse endoprosthesis is, however, unclear. We measured the motion pattern of 16 patients (18 shoulders) during three active and passive range of motion tasks using a six degree-of-freedom electromagnetic tracking device. Despite rotator cuff deficiencies, glenohumeral elevation contributed roughly two-thirds of the total thoracohumeral elevation, which is comparable to healthy subjects. However, patients could not actively use the full range of motion provided by the prosthesis. Although we found considerable interindividual differences in shoulder kinematics, the limitation in glenohumeral range of motion appears related to a lack of generated muscle force and not the design of the prosthesis. Level of Evidence: Level IV, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence. © 2008 The Association of Bone and Joint Surgeons.