The study reported in this article draws upon data collected for the programme "Increasing the Efficiency of Mathematics and Science instruction" (SINUS-Transfer), a professional development project in Germany. This programme's approach requires teachers to improve their teaching in a cooperative manner and with regard to pedagogical problem areas in classrooms. A customized portfolio concept was developed to address the portfolio method as a means of supporting and evaluating teacher professional development. The teachers' acceptance of the method is considered to be the central prerequisite for successful use of the portfolio; therefore, the main question investigated in this paper is how teachers assess the portfolio. Results of latent-class analyses show that acceptance of the portfolio differs widely for different groups of teachers. The study provides indicators on requirements that have to be met in order to effectively use the portfolio as a tool for evaluation. (Contains 3 figures.)