Despite an unprecedented increase in classroom-based support staff, there are confusing messages about their appropriate deployment and a lack of systematic evidence on their impact. This article addresses the deployment and impact on pupil engagement and individual attention of support staff, commonly known as teaching assistants (TAs), in terms of: (1) a comparison between TAs and teachers; (2) differences between pupils with and without special educational needs (SEN); and (3) differences between primary and secondary schools. Systematic observations of pupil behaviour in 49 primary and secondary schools showed that support staff presence resulted in increased individualisation of attention and overall teaching, easier classroom control, and that pupils showed more engagement and a more active role in interaction with adults. This supports teachers' positive view of support staff, but their presence also meant pupils' contact with teachers declined and at secondary level there was less individual and active interactions between teachers and pupils. (Contains 17 tables.)