The Effective Pre-school and Primary Education Project 3-11 (EPPE 3-11) involves a number of components of "Tiers" of research. Tier 1 involves the analysis of primary school effectiveness across all primary schools in England using value added approaches (Melhuish et al, 2006). Tier 2 focuses on following up the academic and social/behavioural progress of children in the original pre-school sample across Key Stage 2 of primary education (age 7 to 11 years). In the original EPPE research children were tracked from age 3 years to the end of Key Stage 1, at aged 7 years plus (see Appendix O for the full range of EPPE Technical Papers). Tier 3 focuses on variations in classroom practice during Key Stage 2 focusing on Year 5 classes. It involves a sample of 125 schools and classes from among the 850 plus schools in which the EPPE children were located. This is the first paper in a series reporting on the classroom observations component of the study Tier3). It presents results of the analysis and comparison of classroom observations conducted in Year 5 classes in 125 primary schools during the spring and summer terms of 2004 and 2005. The paper provides a description of the sample of schools and details of the two observation instruments used. Interest centres on the extent to which the instruments identify variation between classes in different aspects of teachers' practice and in children's observed responses. In addition, analyses are described that explore the associations between several Ofsted measures of overall school quality and effectiveness ('improvement', 'teaching and learning'), and the observed measures of teachers' behaviour and children's responses. Further analyses also linking classrooms observations to value added indicators of school effectiveness derived from the Tier 1 component of the research using national assessment data are also described.