Several converging lines of contemporary evidence suggest that weakness presents a more serious compromise to movement function in poststroke hemiplegia than spasticity. This review examines the clinical and functional phenomena of weakness in poststroke hemiplegia, currently available evidence identifying physiologic substrates contributing to weakness, and reports of early investigations involving high-resistance training targeted at improving strength and the transfer of strength to improvements in functional capacity. Based on this information, we describe some unsolved problems and indicate some likely lines of development to increase our knowledge regarding how resistance training can be included in effective stroke rehabilitation.