The ability to use tools skillfully is generally regarded as one of the major achievements in the evolutionary development of the human nervous system. It is possible for controlled movements of muscles to be executed only if sensory information is integrated into complex neural circuits at various hierarchical levels. The chapters in this volume deal with basic and clinical aspects of integrative processing of sensory and motor activities. New findings emphasize the important influence of somatosensory activity such as tactile, proprioceptive, noxious cutaneous, and articular input on motor output. Furthermore, recordings of evoked potentials as well as unit recordings indicate that sensory and cortical activities are highly interrelated. Control of muscles by motoneurons is exerted both electrically and chemically. Disturbed muscle-motoneuron interaction is reflected in ultrastructural motoneuron morphology and may be of importance in the pathogenesis of motoneuron disease. Long loop reflex testing under various pathological conditions provides insight into disturbed sensory motor circuitry in humans. Electrophysiological recording as well as neurochemical and im­ munohistochemical studies elucidate the neural circuitry of basal ganglia and their neural connections, thus providing improved therapeutic concepts. The role of the thalamus and thalamocortical connections in sensory motor processing is of particular interest, because motor disturbances such as tremor or dystonia can be effectively relieved by stereotaxic interventions at the subthalamic or thalamic level.