The formative evaluation of university science courses aims to provide information about such courses that can be used to facilitate their improvement. An account is given of examples of the formative evaluation of three first and second year undergraduate courses in physics and biology. These case studies provide the basis for discussion of some key issues in formative evaluation. They are the role of the evaluator, the relationship between evaluators and university teachers, and the kind of evidence that is valid for course improvement. The case studies are also examined with respect to the style of evaluation that was the most productive. Although the individual aims of each study were met, deficiencies in the evaluative approach were discernible in some cases. An examination of these shortcomings led to a proposal for a style of evaluation called supportive evaluation. This is described in terms of the relationship between the evaluator and teacher and the responsibilities of each party.