The study of deductive reasoning is one of the key areas in cognitive psychology. Much of the initial impetus for studies in this field came from a developmental perspective, in particular as a consequence of Piaget's theory of formal operations. Subsequent research that examined some of the predictions of this theory has led to what appear to be quite contradictory conclusions. On the one hand, reasoning is held to be a very precociously acquired, if not innate, competence, such that very young children can reason 'logically'. On the other hand, reasoning in adults has been found to be so convincingly 'illogical' that the very notion of logical competence, even in educated adults has been put into question. Clearly, these two conclusions cannot both be true, and their continuing existence reflects the increasing fragmentation of studies of children and of adults. In this context, developmental studies appear to be critical to an integrative approach to reasoning that considers both children and adults within a single theoretical and empirical perspective. This issue presents some of the more important developmental perspectives on the development of reasoning.