This report reviews international research on the impact of early years provision upon young children. Emphasis is given to work related to disadvantaged children. The issues of timing, duration, type, quality and quantity of early years provision are considered in terms of developmental effects upon children and when possible parents. An evaluative summary of the literature on cost benefit analyses of early years provision is also included. Conclusions tempered by the relative rigour and extensiveness of the evidence are produced. Early research was primarily concerned with whether children attending institutions developed differently from those not attending such centres. Later work recognised that childcare is not unitary and that the quality or characteristics of experience matters. Further research drew attention to the importance of the interaction between home and out of home experience. High quality childcare has been associated with benefits for children's development, with the strongest effects for children from disadvantaged backgrounds. There is also evidence that sometimes negative effects occur. The studies have largely been American but research elsewhere, including the UK, indicates results are not culture-specific. While the research on pre-school education (3+ years) is fairly consistent, the research evidence on the effects of childcare (0-3 years) upon development has been equivocal with some studies finding negative effects, some no effects and some positive effects. Discrepant results may relate to age of starting and also probably at least partly to differences in the quality of childcare received by children. In addition childcare effects are mediated by family background with negative, neutral and positive effects occurring depending on the relative balance of quality of care at home and in childcare. Recent large-scale studies (EPPE, NICHD) find effects related to both quantity and quality of childcare. The effect sizes for childcare factors are about half that for family factors. However, family effects incorporate genetic factors. Hence, family and childcare effects may be more equivalent than this comparison implies. Family factors and childcare quality covary, low-income families tending to have lowest quality care. The analysis strategy of most studies attributes variance to childcare factors only after family factor variance has been extracted. Where the two covary this will produce conservative estimates of childcare effects.