This chapter focuses on the economic analysis of what has been called the age of mass migration, 1850 to 1913, and its aftermath up to 1940. This has captured the interest of generations of economic historians and is still a highly active area of research. Here we concentrate on migration from Europe to the New World as this is where the bulk of the literature lies. We provide an overview of this literature focusing on key topics: the determinants of migration, the development of immigration policy, immigrant selection and assimilation, and the economic effects of mass migration as well as its legacy through to the present day. We explain how what were once orthodoxies have been revisited and revised, and how changes in our understanding have been influenced by advances in methodology, which in turn have been made possible by the availability of new and more comprehensive data. Despite these advances some issues remain contested or unresolved and, true to cliometric tradition, we conclude by calling for more research.