This paper discusses the value and place of evaluation amidst increasing demands for impact. We note that most informal learning institutions do not have the funds, staff or expertise to conduct impact assessments requiring, as they do, the implementation of rigorous research methodologies. However, many museums and science centres do have the experience and capacity to design and conduct site-specific evaluation protocols that result in valuable and useful insights to inform ongoing and future practice. To illustrate our argument, we discuss the evaluation findings from a museum-led teacher professional development programme, Talk Science.