This special issue examines the relationships between cognitive systems for perception and action. ''Action'' is meant in a very broad sense, to include processes of selecting, planning, and executing overt responses. Recent interest in the psychology and neuropsychology of action has led to a variety of approaches describing visuomotor systems and the relationship between perception and action. There is at least one basic constraint on this relationship that everyone can agree on: Perceptual systems have evolved to guide action. So although it is at least conceivable that perceptual processing could go on largely independent of concurrent action, a system that planned and executed actions without perceptual guidance would be worse than useless - it would be a complete disaster in anything but the most predictable environment.