This chapter identifies and describes weakness of the upper extremity (UE) through exploration of impairment in movements following cerebral injury. Weakness can result from inadequate strength caused by limitations in force production. Weakness is recognized as a major impairment causing disability and thus a primary obstacle to stroke recovery. Prognosis of UE motor recovery after upper motor neuron (UMN) lesions remains difficult and uncertain. In light of reduced health care reimbursement, development of accurate prognostic indicators is essential for the delivery of cost-efficient rehabilitation services and to guide clinicians in appropriate allocation of patients' resources. Future clinical outcome research studies should evaluate the degree to which the UE treatment intervention that affects weakness also affects functional outcomes as well as family functioning and quality of life. This constellation of data will inevitably lend greater insight into the role and value of UE strengthening in neurorehabilitation.