This report presents the results of analyses related to the Key Stage 2 phase of a major longitudinal study investigating the influence of pre-school and primary school on children's cognitive and social/behavioural development (EPPE 3-11) in England. The study is funded by the Department for Education and Skills (DfES). The focus of this report is on children's cognitive attainments at the end of Year 5. A report on children's social/behavioural development at this age has been published separately. The original EPPE pre-school sample was recruited to the study at age 3 years plus and followed to the end of Key Stage 1 (Year 2) in primary school. An additional 'home' sample of children (who had not attended pre-school) was recruited at the start of primary school. The EPPE 3-11 project is following up the whole (the pre-school and 'home') sample to the end of primary schooling (age 11 years plus). The research has adopted an educational effectiveness design and mixed methods approach (Sammons et al., 2005; Siraj-Blatchford et al., 2006) in order to investigate child, family and home influences on developmental outcomes so that the relative importance of these influences can be studied in relation to the strength of pre-school and primary school factors.