This monograph concerns the psychological processes underlying the development of executive function, or the conscious control of thought and action. It has long been clear that these processes change considerably in early childhood, transforming a relatively stimulus-driven toddler into a child capable of flexible, goal-directed problem solving. However, the nature of these processes has remained elusive. In a programmatic series of 9 experiments, the authors examine circumstances that help or hinder executive function in 3- to 4-year-old children. The results provide the basis for a revision of their Cognitive Complexity and Control (CCC-r) theory, according to which there are age-related increases in the complexity of the rules that children can formulate and use when solving problems. The revised theory (a) specifies more clearly the circumstances in which children will have difficulty using rules at various levels of complexity, (b) provides a more detailed account of how to determine the complexity of rules required in a task, (c) takes account of both the activation and inhibition of rules as a function of experience, and (d) highlights the importance of considering intentionality in the study of executive function.

Philip David Zelazo (Ph.D. 1993, Yale University) is Associate Professor of Psychology and Psychiatry at the University of Toronto, where he holds a Canada Research Chair in Developmental Neuroscience. His research concerns dorsolateral prefrontal and orbitofrontal contributions to the typical and atypical development of executive function. He is Editor of the Journal of Cognition and Development, and Series Editor of the Developing Mind Series.

Ulrich Müller (Ph.d. 1998, Temple University) has been Assistant Professor Psychology at Pennsylvania State University, and is now at the University of Victoria, in British Columbia, Canada. His research centers on the early development of reasoning, executive function, and social understanding. He serves on the Editorial Boards of Child Development and the Journal of Cognition and Development and has been a member of the Board of Directors of the Jean Piaget Society.

Douglas Frye (Ph.D. 1978, Yale University) is Chair of the Psychology in Education Division, Graduate School of Education, University of Pennsylvania. His research focuses on children's theory of mind, executive function, and strategies in mathematical understanding. He is currently Associate Editor of the journal Child Development and serves on the Editorial Board of the Journal of Cognition and Development.

Stuart Marcovitch (Ph.D. 2001, University of Toronto) is currently a postdoctoral fellow in the Department of Infant Development, New York State Institute for Basic Research, Staten Island, New York. His research focuses on cognitive development in infants and preschoolers.