Background: Over the past 20 years or so policy and practice on the education of children with special educational needs (SEN) has been aimed at placing increasing numbers of children in a mainstream school environment. Although this policy has been supported in principle by many teachers, parents and local authority officers, there has been much less agreement about whether this principle can be realized in practice, and even if it can, about what the impacts might be on the achievements of pupils with SEN in mainstream schools and, in particular, on their peers. Purpose: This paper discusses the key findings from a systematic review of the literature carried out by the Inclusion Review Group, on behalf of the Evidence for Policy and Practice Information (EPPI)-Centre, the purpose of which was to review research evidence on whether the placement of pupils with special educational needs (SEN) within mainstream schools has an impact on academic and social outcomes for pupils "without" SEN. Design and methods: The methodology followed the procedures adopted by the EPPI-Centre. Having agreed on the inclusion and exclusion criteria for studies that could be included in the review, an initial pool of 7137 papers were identified through electronic databases. After having screened all their titles and/or abstracts and having marked out possible papers to be included in the review, 119 paper copies were obtained--all of which were read by one or more of the authors of this paper. This led to a further reduction to 26 studies that were subjected to the EPPI data extraction process and synthesis. Conclusions: Overall, the findings suggest that there are no adverse effects on pupils without SEN of including pupils with special needs in mainstream schools, with 81% of the outcomes reporting positive or neutral effects. Despite concerns about the quality of some of the studies that were reviewed and the fact that the great majority were carried out in the USA, these findings should bring some comfort to headteachers, parents and local authority officers around the world at a time when concerns have been raised about the problems that schools face in responding to the twin agenda of becoming more inclusive and, at the same time, raising the achievements of all their pupils. (Contains 3 tables.)