Economic development requires people to move to where the jobs are, from lagging to leading regions within a country or across borders. This leads to optimal utilization of their human capital and to important gains for them and the economy. The movement of migrants to economic opportunities and to networks that help them integrate the labor market, leads to geographical concentrations of migrant populations. Notably the flows of high-skilled migrants are very concentrated, as they tend to go to few countries worldwide, and to a few selected areas within the country. Agglomeration effects and knowledge spillovers increase the productivity of high-skilled workers who work in the same area or collaborate with other high-skilled workers.