The foundations of brain architecture are established early in life through a continuous series of dynamic interactions in which environmental conditions and personal experiences have a significant impact on how genetic predispositions are expressed. Because specific experiences affect specific brain circuits during specific developmental stages--referred to as sensitive periods--it is vitally important to take advantage of these early opportunities in the developmental building process. The quality of a child's early environment and the availability of appropriate experiences at the right stages of development are crucial in determining the strength or weakness of the brain's architecture, which, in turn, determines how well he or she will be able to think and to regulate emotions. This report summarizes in clear language the most recent scientific advances in understanding the importance of sensitive periods on brain development, and the implications of those findings for policy. (Contains 2 figures.)