This chapter reviews sensitive periods in human brain development based on the literature on children raised in institutions. Sensitive experiences occur when experiences are uniquely influential for the development of neural circuitry. Because in humans, we make inferences about sensitive periods from evaluations of complex behaviors, we underestimate the occurrence of sensitive periods at the level of neural circuitry. Although we are most interested in complex behaviors, such as IQ or attachment or externalizing problems, many different sensitive periods at the level of circuits probably underlie these complex behaviors. Results from a number of studies suggest that across most, but not all, domains of development, institutional rearing limited to the first 4-6 months of life is associated with no significant increase risk for long-term adverse effects relative to non-institutionalized children. Beyond that, evidence for sensitive periods is less compelling, meaning that "the earlier the better" rule for enhanced caregiving is a reasonable conclusion at the current state of the science.