Background: The increasing numbers of teaching assistants (TAs) and their widening role in English primary classrooms is part of wider international developments. Several large quantitative studies have been influential in shaping debate, but little qualitative research exists on TAs and the content of their activity in the key area of mathematics. Purpose: The article addresses the question: How do TAs in primary schools perceive their role in daily mathematics lessons? Sample: Twenty-four TAs employed in English primary schools participated in the study. Their experience ranged from 2 to 20 years. Two were men; the rest were women. Design and methods: Interviews were undertaken in the spring and summer of 2011, transcribed and coded using Glaser and Strauss' recommended method (B. Glaser and A. Strauss, 1967, "The discovery of Grounded Theory: Strategies for qualitative research"; Chicago, IL: Aldine Publishing Company). A typology of TA activity was created and used together with other cross-cutting factors to ascertain what TAs did in general and, specifically, in mathematics teaching. Results: The results show that previous pictures generated from large-scale quantitative studies of TAs working with children seen as having the greatest difficulty in mathematics are essentially (though not universally) correct. However, they may be internally differentiated to a greater extent than previously envisaged. Conclusion: The range of TA activities in mathematics classrooms is considerably wider than previously thought; professional development for TAs and teachers needs to take account of this.