Swiss parliamentary party groups have undergone a process of professionalization over the course of the last few decades. Swiss parties increasingly resemble party groups in established and more professionalized Western European parliamentary systems. Party unity has increased and party leaderships have started using instruments to strengthen party unity, in view of which party group members increasingly accept behavioural rules and norms. Our analysis suggests that Swiss parties have professionalized over the course of the last thirty years, and we demonstrate that this change has resulted from the ideological polarization in the Swiss political landscape, and conscious effort on the part of Swiss party group leaders. Thus, we contribute to the on-going debate about party change and the question of what drives the development of parties. The development of the Swiss parties is documented using data from three parliamentary surveys from the last thirty years, and two interview rounds with party group leaders, their staff and political experts.