This book focuses on the functions of corticospinal projections in the primate brain. Recent observations concerning the details of the cortico-cortical connections which contribute to the determination of these functions are presented in this volume. The details of cell-to-cell connectivity which allows corticospinal neurones to influence selectively the behaviors of individual motor units in the hands of both monkeys and humans are also covered. The experimental observations are dealt with against an historical background of histological and electrical examination of the motor areas of the cerebral cortex of humans, and the clinical significance of recent observations is discussed in connection with studies of the functions of the human brain during voluntary execution of movement, revealed by such techniques as positron emission tomography (PET). Neuroanatomical and neurophysiological details are correlated with measures of dexterity in movement performance and also used to account for the deficits in movement control which follow stroke, the learning of skill in movement performance and the rehabilitation of movement capacity after brain injury and disease.