This paper reports on a research project that set out to address three contemporary concerns and to explore the relationships between them. These are: (i) the concept of achievement, (ii) meanings of inclusion, and (iii) the use of evidence to inform educational policies and practices. The relationship between inclusion and achievement is important because some schools continue to resist becoming more inclusive on the grounds that that doing so will have a negative effect on the academic achievement of other students and will lower overall standards. This paper presents a critical examination of available evidence of this relationship and offers guidance on how to undertake research on this complex topic. The paper provides a theoretical discussion about the nature of educational inclusion and achievement and how these concepts might be examined through contrasting yet complementary methodological approaches.Research on inclusion and achievement that has an over-reliance on some types of quantitative evidence may restrict the outcomes of research, because the analysis of such evidence may promote a narrower view of both achievement and inclusion. That is, achievement may be reduced to performance scores in core curriculum subjects, thus disregarding achievements relating to others areas of the curriculum and aspects of children's lives. Similarly, inclusion may be restricted to focusing on the perceived learning difficulties of a small number of individual children, rather than looking more broadly at the contexts in which all teaching and learning takes place. Therefore the emphasis must be on gathering evidence which is appropriate to the research focus and useful to the researcher.The research reported in this paper used both quantitative and qualitative data. First there will be a brief review of the possibilities and problems of using evidence from large-scale national data sets. This will be followed by an overview of the 'Framework for Participation', a systematic method for collecting detailed contextual evidence to explore the relationship between inclusion and achievement at the level of individual students, classes and schools, whilst taking into account the influence of broader issues of school cultures and values and beliefs. This will be illustrated with reference to multi site analysis of four case study schools.The paper will offer pragmatic and structured guidance to support those who may wish to research inclusion and achievement. One of the outcomes of the project was a series of questions and suggestions to support practitioners to reflect on how they might collect and interpret evidence in ways which are 'systematic, critical and self-critical', so as to advance their knowledge about developing the achievements and the inclusion of children and young people with whom they work. These suggestions are not intended to provide 'quick fixes', but to promote greater understanding as a condition for bringing about sustainable change. As is clearly illustrated by the four case studies, not only is the relationship between inclusion and achievement far too problematic for simplistic solutions, but schools and classrooms are complex worlds shaped by the diverse and shifting social, emotional and cultural lives of the staff and students who inhabit them. Black-Hawkins, K., Florian, L. & Rouse, M. (2007, in press) Inclusion and Achievement in Schools. London: Routledge Stake, R. E. (2006) Multiple Case Study Analysis. London: The Guilford Press. Co-authored book