In a project-organized problem-based learning curriculum, students are encouraged to learn through a process where they write questions and find their own answers. Their supervisor has an important function in facilitating this process. When it comes to project assessment, this pattern is often changed to one where the supervisor asks the questions and the students are to answer them. From a constructivist point of view, this may seem inappropriate in relation to students' learning. Therefore, a new approach to project assessment was developed in a longitudinal case study at Aalborg University based on Niklas Lumann's operative constructivism. The research was based on participatory action research. According to that paradigm, the researcher must solve the task of supervising and assessing the students at the same time as trying to understand the ongoing learning process. In that way, a coherent model for supervision and assessment with emphasis on competence development was developed. According to the model, the students must both ask and answer questions during the project assessment. The assessors facilitate the process and should ask questions only if the students are obviously lacking essential understanding of their own project. This model was tested during five assessments of the engineering study at Aalborg University. The result was that the students were more motivated and they had learned how to reflect their own learning process and how to take responsibility for their own learning. (Contains 4 figures.)