First published in 1977. The present series of essays, of which this is the first volume, attempts to describe what is going on in a particular speciality in such a way that it can be easily assimilated by workers in other branches of psychology. The essays do not provide comprehensive reviews of specialized topics: They are intended to convey new concepts and new approaches without covering in exhaustive detail all the relevant experimental work. They should be intelligible to any psychologist regardless of his field and also to the advanced undergraduate student.