The goal of this study at Blythedale Children's Hospital is to test the efficacy of transcranial direct current stimulation (tDCS) and robotic upper extremity therapy in improving upper extremity function in children with unilateral cerebral palsy. This study will test the hypothesis that physical rehabilitation, provided by repetitive arm movements guided by a robot, will improve upper extremity function in children with hemiplegia, and that this improvement can be enhanced by transcranial direct current stimulation of motor cortex immediately before robotic training.