Teaching Assistants (TAs) in primary schools in England have a growing pedagogic role. For some, this sometimes includes responsibility for the whole class instead of the teacher. This article draws on 24 interview transcripts to examine the practice in the context of primary mathematics lessons and from TAs' viewpoints. Emergency cover is often seen as reasonable where good working relationships exist. The practice of being regularly responsible for mathematics lessons evokes more diverse reactions. Some TAs initially appear to support the "official" view that it is unproblematic to run a lesson from pre-prepared plans, though close inspection reveals a different picture. Others acknowledge that the interactions involved in such lessons are not necessarily susceptible to planning. The findings raise considerable doubt about current policy and question its presentation as a way to raise standards.