This chapter reviews existing research analysing the relationship between immigration and welfare states. This relationship is analysed in both directions, namely how immigration affects the welfare state, and how the welfare state affects migration. The chapter focuses on 6 prominent research questions in the field, namely 1) how different welfare regimes accommodate immigrants 2) how immigration may affect political support for the welfare state 3) the fiscal effects of immigration on the welfare state, 4) whether migrants make residence decisions based on welfare (‘welfare magnet hypothesis’), 5) whether the public supports excluding migrants from welfare (‘welfare chauvinism’) and 6) the extent to which welfare states depend on immigrant labour. The chapter then draws on available evidence to highlight two relevant research agendas for the future.