"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. Of the 990,553 foreign nationals admitted to the United States in FY2013 as lawful permanent residents (LPRs), 649,763, or 66%, were admitted on the basis of family ties. Of these family-based immigrants admitted in FY2013, 68% were admitted as immediate relatives of U.S. citizens. Many of the 990,553 immigrants were initially admitted on a temporary basis and became immigrants by converting or 'adjusting' their status to a lawful permanent resident. The proportion of family-based immigrants who adjusted their immigration status while residing in the United States (54%) exceeded that of family-based immigrants who had their immigration petitions processed while living abroad (46%), although such percentages varied considerably among the five family-based admission categories."
CRS Report for Congress, R43145
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html