Family reunification is a key principle underlying U.S. immigration policy. It is embodied in the Immigration and Nationality Act (INA), which specifies numerical limits for five family-based admission categories, as well as a per-country limit on total family-based admissions. The five categories include immediate relatives of U.S. citizens and four other family-based categories that vary according to individual characteristics such as the legal status of the petitioning U.S.-based relative, and the age, family relationship, and marital status of the prospective immigrant.