This contribution addresses the question of competences and skills for public professionals to improve collaboration. The authors start their search by looking for a conceptual focus. Poor collaboration is conceptualized as ‘colliding rationalities’. Herbert Simon has introduced the concept of ‘bounded rationalities’ to oppose scientists who think that improvement of public administration is possible through ex ante theorizing. Human beings not only have a limited capacity for rational thinking, but there also multiple rationalities that restrict each other. A classic distinction is made by Ig Snellen: a) legal rationality, b) political rationality, c) financial/economic rationality, d) scientific rationality. Not only are rationalities restricted, but also collisions can occur between professionals who adhere to different rationalities. These collisions have to do with their relational network, with the nature of their tasks and with their way of thinking and communicating. Therefore the authors have worked with different groups of professionals to find out which competences they need to deal with these collisions or make them productive. They did so on the basis of specific cases in their daily working practice. Jan Fraanje comments on this contribution as a practitioner in a Dutch municipality.