How do employment relations change even though the formal structure of employment relations institutions remains stable? This dissertation draws on an analysis of collective bargaining in three multinational auto companies in Germany, Spain and the United States to answer this question. The study traces the longitudinal changes of employment relations in each country as well as the emergence of similar employment relations practices across countries. Previous literature on comparative employment relations linked formal employment relations institutions to national patterns of employment relations. In contrast, this study argues that the ideas and ideologies of management and labor underpin the functioning and meaning of institutions. As the collective actors adapt to a changing socio-economic context, their ideas and ideologies change, which contributes to an evolution of institutional practices and a different enactment of institutions. First, management and labor make different use of their institutional rights and resources, e.g. they can apply forcing strategies with varying intensity and develop contentious, cooperative or market-oriented employment relations in identical institutional settings. Second, despite institutional constraints, the collective actors have a lot of leeway to develop new employment relations practices and instruments, which then in turn can alter the functioning and meaning of institutions. Institutions are what actors make of them. This dissertation is a contribution to the literature on institutional change and comparative employment relations. Previous institutional literature assumed a rational foundation of actor behavior and focused on formal institutions. In contrast, this study suggests a constructivist institutionalism, which analyzes the mutual relationship between ideas and institutions. The dissertation inductively explored five mechanisms for ideational change that contribute to institutional change: leadership change, identity work, mimicking and learning, collective bargaining and generational change.