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.