The traditional view of motor systems as a linear chain of elements switched on and off by command neurons has become increasingly difficult to maintain in the face of accumulating evidence against the existence of command elements. So far, however, the general formulation of an alternative approach has been lacking. This book, by summarising the evidence against the linear approach to motor systems, argues forcefully against it. Analyses are presented of motor systems ranging from the lobster stomatogastric system through molluscan systems, leech movement, insect singing and locomotion, fish and amphibian behaviour, to goal-directed a movements in primates and volitional movements in humans. Comparison of these motor systems reveal the existence of some general principles underlying motor control and behavioural choice such that motor systems appear generally to be parallel, distributed processing networks. By discussing the treatment of motor systems in terms of parallel distributed processing systems, this book presents in concentrated form an alternative to the earlier view of motor systems.