This review summarizes the conclusions reached by recent studies of immigration to the United States. The central questions that motivate this literature are: How has the character of immigrants changed over time? How have these immigrants been assimilated and performed economically in the United States? What demands have immigrants placed on public services and income transfer programs? What consequences have immigrants had for native-born workers? These questions span a large field of study, and references and discussion of major issues of interpretation and uncertainty are of selective necessity. The first section of this paper discusses how characteristics of cohorts of immigrants that enter the United States at different times can be compared, both with earlier or later immigrants and with native-born Americans. The second section describes the conclusions from studies that attempt to quantify the assimilation process, where the standard practice is to compare the economic productivity or earnings of immigrant and native workers, although consideration of how well the children of the immigrants are assimilated might bring us closer to the long-run effects of immigration. Section three discusses the social consequences of legal and illegal immigration and the unresolved problems we have in assessing these consequences from local labor market studies. Finally, the characteristics and assimilation of immigrants can be affected by immigration policies that can emphasize family reunification or skill-based selection of immigrants or humanitarian assistance to refugees. Policies that reduce the flow of legal immigration can also influence the flow of illegal immigration to those seeking a refugee status. A final section recapitulates the findings.