Shared expertise, team teaching, and cooperation among lecturers from different fields have become more and more important in promoting learning and achieving more innovative learning outcomes in multidisciplinary universities. To increase and improve sharing expertise between teachers from different faculties and disciplines, we wanted, on one hand, to identify skills and competences that teachers have in common and, on the other hand, to find areas in which they identify that they need complementation. As a framework for this research, we applied Lee Shulman's (1986) seven categories of teachers' knowledge base including the theory of pedagogical content knowledge (PCK). The data were collected by group discussions. The teachers (N = 22) represented all seven faculties of Helsinki Metropolia University of Applied Sciences (UAS), that is, Business School, Civil Engineering and Building Services, Culture and Creative Industries, Health Care and Nursing, School of Information and Communication Technology (ICT), Industrial Engineering, and Welfare and Human Functioning. The data were analyzed using theory-based content analysis. According to our data, the mutual core competence of a teacher is the capacity to interact effectively. It is a basis for shared expertise. Interaction skills are necessary in collaborative construction of knowledge as students, teachers of different fields, and their partners inside and outside the organization co-operate. Multidisciplinary co-operation among colleagues also helps to maintain subject matter knowledge, as it supports peer learning and encourages everyone to move out of their comfort zones.