The recovery of voluntary arm movements is one of the most important goals during stroke rehabilitation in order to avoid long-term disability in activities of daily living. Devices that are currently used for rehabilitation and training in clinical settings are mostly bulky and stationary. One of the objectives of the European project RETRAINER is to develop a lightweight arm exoskeleton that can be used for clinical rehabilitation as well as home training and is suitable for movement control by means of neuromuscular electrical stimulation (NMES). The RETRAINER arm exoskeleton represents a passive, 3-degrees of freedom (DoFs), lightweight arm orthosis with gravity compensation at the shoulder and elbow joints to minimize muscular effort during arm movements. All DoFs can be locked via electromagnetic brakes to keep static postures without continuous muscle work. Main focus of the development was the modularity and easy adaption to a wide range of user’s anthropometrics as well as increasing the user’s comfort while wearing the exoskeleton. A universal mounting device allows to install the exoskeleton on both wheelchairs and normal chairs with different backrest shapes. The exoskeleton can be used together with NMES of a maximum of two arm muscles. NMES is triggered by means of the residual muscle activity of the subject, so as to enhance motor recovery. First tests of the prototype with healthy subjects showed promising results regarding functionality and wearing comfort. Further prototypes are currently in two rehabilitation centres for starting test series with stroke patients.