Researchers have long been interested in why some children have behavioral and developmental problems while others appear to develop "normally." A substantial body of research supports the notion that a child's early experiences--from birth to school entry--can influence significantly later cognitive, behavioral, educational, and economic outcomes. This review of the literature focuses on factors that affect children during their first 5 years of life. The review begins by examining the literature that identifies associations between children's outcomes and their early family and neighborhood experiences. This literature forms the basis for past efforts to initiate early childhood intervention programs (such as home visiting, childcare, and preschool). Next, the review examines recent discussions of research into brain development, especially as it relates to cognitive and behavioral development before age 5. These findings provide support for early childhood intervention programs. The third section of the review examines evaluations of early childhood intervention strategies for families with infants and young children; the fourth section reviews early childhood programs operated by federal and state governments in California. The final section identifies a number of policy issues and options the legislature may wish to consider when debating early childhood intervention proposals. Appendices present trends in child indicators, basic structure of the brain at the cellular level, and experimental research on cognitive development. Contains about 140 references. (EV)