Teaching assistants (TAs) comprise a quarter of the school workforce in England and Wales. There has been controversy over TAs' deployment and appropriate role regarding supporting learning and these debates have been transformed by findings from the largest study of school support staff (the DISS project), which show that TA support has a negative impact on pupils' academic progress. This article conceptualises the most likely explanations for the negative effects of TA support in the form of the wider pedagogical role model, the components of which enable us to understand the effects of TA support in terms of the decisions made about TAs, rather than by them. (Contains 9 notes, 3 tables, and 1 figure.)