Amidst heated debates on immigration and “migrant integration”, the European Union becomes an increasingly relevant actor, where important resources are earmarked for the implementation of civic integration measures, as well as for producing “scientific evidence” to guide policy. Simultaneously, a prolific scholarship attempts to understand, measure and compare how and whether immigrants are “integrated into society”, often in the effort to remain “policy-relevant”. This study joins other critical works that draw attention to the ways “integration” is debated, legislated, conceptualized, monitored, evaluated, and ultimately, normalized as a mode of governance. Situated at the interstices of migration studies, European studies, and the social studies of science, the dissertation examines the role of scientific research, EU policy, and research-policy knowledge infrastructures in shaping the “immigrant integration” paradigm in Europe. Interested primarily in integrationism as a technique of power, I take a decolonial and genealogical approach that situates integrationist discourses within wider and intersecting systems of hierarchy. The main argument is that the politics of integration research and the scientific claims in “evidence-based” policy intersect to produce “migrant integration” as the hegemonic paradigm in governing migration-related diversity in Europe. Through discourse analysis of research publications, policy documents, media statements, as well as a virtual ethnography of the EU’s science-for-policy community, I examine how integration comes to be seen simultaneously as a political problem and an object of scientific fascination, how is integration regulated at supranational level and through science-policy collaboration, and what are the power effects of integrationism, as a rationality of governance, on its target subjects. Ultimately, I argue, the practices of regulating, governing, measuring, theorizing and monitoring the integration of immigrants are shaped by power relations linked to the preservation of European liberal subjecthood against rapid demographic, social, political, and environmental shifts.