In many federal states, subnational governments are increasingly involved in immigration policymaking, which has resulted in powers and resources decentralization, and policy development at the subnational scale. This project aims at opening the black box of the vibrant subnational policy variation when it comes to immigration and immigrant integration matters in 4 federal states (Belgium, Canada, the United States, and Switzerland). The innovative character of this research is to look more closely at the subnational level, taking into account the diversity and the asymmetry that exist inside each federation. Contributing to immigration federalism literature and immigration studies, this project pursues a twofold objective. Firstly, it strives to gather and organize original data regarding 89 subnational units’ involvement in different aspects of immigration/immigrant integration policymaking. Using a multidimensional grid, the extensive data gathered will be systematized and operationalized thanks to an index (SIPIX). Secondly, building upon a comparative research design and using a fuzzy-set Qualitative Comparative Analysis, this project aims at investigating the factors (‘conditions’) determining the subnational entities’ involvement in immigration/immigrant integration policymaking (‘outcome’). By its originality and its extensiveness, this research will both offer rich new scientific material and provide fruitful insights to understand the immigration-federalism nexus.