This paper considers the key findings of a DfES-funded study that explored the relationship between achievement and inclusion in mainstream schools in England. The methodology involved a statistical analysis of nationally held data on all pupils at the end of key stages 1-4, together with a series of site visits to "inclusive" mainstream schools. These were carried out in order to explore school processes that might explain how some schools can manage to be both highly achieving and inclusive. Findings indicate that there is a small, but for all practical purposes, insubstantial relationship between inclusion and academic achievement at the school level although there is also a large degree of variation suggesting strongly there are other factors within a school's make up, rather than its degree of inclusivity, that impact on the average academic achievements of its pupils. This was confirmed from the analysis of the school site visits. The overall conclusion, therefore, is that mainstream schools need not be concerned about the potentially negative impact on the overall academic achievements of their pupils of including pupils with SEN in their schools. (Contains 1 figure and 1 table.)