Interdisciplinary co-operations and collaborations become more and more ubiquitous in product development. Besides communication and coordination, especially the partly integration of team members' rather separate fields of knowledge is essential for the success of the product development process. In this paper, we describe a model of interdisciplinary co-operation and a questionnaire to assess co-operation problems. The co-operation questionnaire was designed to help project managers in running their projects. By monitoring their teams, they get hints for necessary improvements in the project management and can better concentrate on the product development process. The application of the questionnaire is illustrated by describing several studies and their results.