The conscious control of behavior is a critical component of cognitive and social functioning throughout the lifespan. The "hierarchical competing systems" model (Marcovitch & Zelazo, 1999) embodies a developmental approach to studying the mechanisms that underlie conscious control. According to the model, increased amounts of experience within a particular context lead to an increased likelihood of reflection, which enables conscious control of behavior. Three studies that are variants of the classic A-not-B search procedure will be presented. The results from these studies strongly implicate the role of conscious control in performance on a variety of tasks in infancy and the preschool years. Future directions will be discussed, including computational modeling of conscious control, serial order learning and the role of language development as a mediator of the conscious control of behavior.