The concept of critical or sensitive periods in child development provides an example around which to organize discussion of what is known and not known about brain development and the implications of brain science on early childhood policy and practice. This book reviews the early history of critical periods and evidence for their existence in various domains of human cognition and learning, including basic sensory systems, social and emotional development, and language learning and acquisition. The book concludes with a section on the relevance of critical periods for early intervention. The chapters are: (1) "A Critical and Sensitive Period Primer" (John T. Bruer); (2) "Developmental Neurobiology Overview: Synapses, Circuits, and Plasticity" (Jeff W. Lichtman); (3) "Critical Periods in the Development of the Visual System" (Jonathan C. Horton); (4) "Critical Periods for Development of Visual Acuity, Depth Perception, and Eye Tracking" (Lawrence Tychsen); (5) "Sensitive Periods in Attachment?" (Ross A. Thompson); (6) "Animal Models of Critical and Sensitive Periods in Social and Emotional Development" (Maria L. Boccia and Cort Pedersen); (7) "Social and Emotional Development in Children Adopted from Institutions" (Elinor W. Ames and Kim Chisholm); (8) "Language Processing: How Experience Affects Brain Organization" (Helen J. Neville and John T. Bruer); (9) "Sensitive Periods in First Language Acquisition" (Heather Bortfeld and Grover J. Whitehurst); (10) "A Critical Period for Second Language Acquisition?" (Kenji Hakuta); (11) "The Subtle Science of How Experience Affects the Brain" (John T. Bruer and William T. Greenough); (12) "Critical Periods and Early Intervention" (Dale C. Farran); (13) "The Concept of Critical Periods and Their Implications for Early Childhood Services" (Robert B. McCall and Bradford W. Plemons); and (14) "Critical Periods: Reflections and Future Directions" (Donald B. Bailey, Jr. and Frank J. Symons). All but one chapter contain references. (KB)