The resurgence of large-scale immigration in many countries has stimulated a great deal of research on many aspects of the economics of immigration. A key insight of economic theory is that the impact of immigration depends on how the skills of immigrants compare with those of natives in the host country. This article examines the ideas and models that are typically used to analyse flows of persons across countries, and illustrates how this framework increased our understanding of the determinants of the direction, size, and skill composition of immigrant flows, and of the consequences of those flows on economic outcomes.