 Transcranial galvanic stimulation tDCS seems to promote motor recovery after stroke by stimulating anodal or inhibiting cathodal neural circuits in the brain. In the treatment of severe arm paresis after stroke, robot-assisted arm training AT proved to be effective, but nevertheless only a few patients could use their affected hand functionally in daily life after robot training. Therefore the present study intends to combine both approaches, tDCS AT, applied at the same time every day for six weeks. The study has three treatment arms, two groups will receive the tDCS, either anodal of the lesioned or cathodal of the non-lesioned hemisphere. The anodal stimulation is expected to facilitate the activity of the arm motor area of the lesioned side directly, while the cathodal stimulation of the non-lesioned hemisphere is expected to facilitate the lesioned side indirectly by decreasing inhibitory inputs. The third group will receive a sham-stimulation. All patients will work with the AT simultaneously to the tDCS, respectively sham-tDCS. 