In this introduction to the special issue, we make the case for a greater use of sociological theories and methods across major approaches in European integration theory, a strategy which we call ‘mainstreaming sociology'. Although sociology as a discipline has, in contrast to political science, shown a late and so far modest interest in the European Union (EU) as a research object, a growing number of scholars is tapping into sociological notions and empirical work conducted by sociologists to expand the realm of concepts, methodologies and terrains available in EU studies. We identify four dominant theoretical approaches in European integration theory to which sociological concepts, methodologies and findings could make a distinctive contribution: constructivism, institutionalism, multi-level governance and intergovernmentalism. Our objective in doing so is not to sketch out a sociology of the EU but to evaluate how useful sociological insights can be incorporated in the political science of the EU.