This article reports the effects of pre-school quality on children's cognitive and behavioural outcomes at age 11 using a large-scale longitudinal study of 3000+ children in England (EPPE/EPPSE). The ECERS-R and a curricular extension to it (ECERS-E) were used to assess the quality of provision in 141 pre-school settings attended by the children. The quality measures were derived from observations throughout the day of interactions and resources related to Literacy, Numeracy and Science learning, as well as observational/interview data related to how each centre catered to diverse needs of children. Multi-level modelling was used to investigate the effects of pre-school quality on children's academic and social-behavioural outcomes at age 11. Pre-school quality significantly predicted most outcomes, after taking account of key child and family factors. More importantly, children who attended low quality pre-schools had cognitive and behavioural scores that were not significantly different from those of children with no pre-school experience. The methods and findings of this large-scale study are considered in terms of the strengths and limitations of "educational effectiveness" designs. It is suggested that mixed methods designs can address many of the limitations. (Contains 9 figures.)