Graduate teaching assistants (GTAs) and postgraduates in the UK are taking on greater responsibility for teaching, and therefore it has become increasingly necessary to explore the teacher training that supports them in this endeavour. This paper outlines an impact evaluation carried out on a graduate certificate programme primarily aimed at GTAs and support staff at a research-intensive institution. The concept of impact evaluation is explored. This qualitative research establishes impact on the approaches participants took and their understanding of teaching following on from the programme. Interviews with nine participants, analyses of their critical reflective portfolios, and analyses of their teaching observation commentary are reported on. Findings reveal participants felt they had gained particular skills of teaching, were more reflective, and adopted a more student-centred approach to their teaching. It is recommended that training on specific teaching skills be incorporated into the provision available for GTAs and that disciplinary differences accommodated.