This article presents an analysis of the association between pupil mobility and educational attainment in the 2002 national end of Key Stage 2 (KS2) tests for 11-year-old pupils in an inner London education authority. The results show that pupil mobility is strongly associated with low attainment in the end of key stage tests. However, the negative association with pupil mobility is reduced by half when account is taken of other pupil background factors known to be related to educational attainment (such as special educational need and socioeconomic disadvantage), and is eliminated entirely when account is also taken of pupils' prior attainment as indicated by end of KS1 test scores at age 7. Thus there is no indication that changing school has a negative impact on educational progress during primary school. Pupils who join their school during KS2 from other schools in England are more likely to be "at risk" of low attainment due to higher levels of socioeconomic disadvantage, a greater need for support in relation to English as an additional language, a higher incidence and greater severity of special educational needs and pre-existing low attainment at the end of KS1. A key factor in understanding the relationship between mobility and attainment is the reason for mobility. One-third of mobile pupils had arrived from schools outside of England, often as refugees, asylum seekers or economic migrants, and these pupils accounted for the major part of the effect ascribed to "pupil mobility." The low attainment of these pupils is the result not of "changing school" but of a broad range of factors including substantial cultural, educational and social adjustment. (Contains 4 tables, 3 figures and 5 notes.)