Portfolios are widely used as instruments for assessment in initial teacher education courses. They are claimed to present a comprehensive picture of student teachers' knowledge and performance. But what type of evidence is needed to safely say that an aspiring teacher has not only grasped essential notions and concepts from the teacher education course, but is also able to implement them in real world classroom situations? This paper reports on the design process of one portfolio assignment that has been developed specifically to capture students' classroom performance and development in their portfolio. Data from the portfolio entries of one student from a small-scale pilot conducted with the assignment are analyzed in detail followed by a discussion of the implications of the case study. (Contains 1 figure.)