Teaching assistants (TAs) are part of a growing international trend toward paraprofessionals working in public services. There has been controversy over TAs' deployment and appropriate role when supporting the learning of pupils with special educational needs (SEN) in mainstream schools. Such debates have been transformed by findings from a large study of school support staff in the UK (the DISS project). The findings from this study show that TA support has a negative impact on pupils' academic progress, especially pupils with SEN. The findings render the current system of support for SEN highly questionable: TAs have inadvertently become the primary educators of pupils with SEN. This paper sets out the likely explanations for the negative effects in terms of three "frames"--deployment, practice and preparedness--and then uses these frames to identify specific implications for pupils with SEN. We offer suggestions on how to make the most productive use of TA support. (Contains 4 tables and 5 notes.)