The Effective Provision of Pre-school Education (EPPE) project explores the impact of pre-school centre provision on young children's cognitive progress and their social behavioural development. The EPPE study was commissioned and funded by the Department for Education and Employment. EPPE Technical Papers 8a and 8b report on the main findings of the first phase of the longitudinal research which tracked a large sample of young children over the pre-school period from age 3 years plus to the start of primary school, when children entered reception classes. An educational effectiveness design was adopted which explores the developmental progress children made during this period and analyses the contribution made by different pre-school centres to cognitive and social behavioural gains.