Between 1840 and 1930, some 37 million immigrants, the overwhelming majority of them Europeans, arrived in the United States. Immigration reached its peak in the first decade of the 20th century, when nearly 9 million came. Following the end of World War I, the tradition of almost unlimited immigration was abandoned, and through the National Origins Act of 1924, a quota system was established as the basis of a carefully restricted policy of immigration. Under the McCarran Act of 1952, one-sixth of 1% of the number of inhabitants from each European nation residing in the continental United States as of 1920 could be admitted annually. In practice, this system favored nations of northern and western Europe, with the UK, Germany, and Ireland being the chief beneficiaries. The quota system was radically reformed in 1965, under a new law that established an annual ceiling of 170,000 for Eastern Hemisphere immigrants and 120,000 for entrants from the Western Hemisphere; in October 1978, these limits were replaced by a worldwide limit of 290,000, which was lowered to 270,000 by 1981. A major 1990 overhaul set a total annual ceiling of 700,000 (675,000 beginning in fiscal 1995), of which 480,000 would be family sponsored and 140,000 employment based.
In 2002, 1,063,732 immigrants entered the United States, of whom 416,860 were subject to the numerical limits. Some 342,099 immigrants in 2002 were from Asia; 404,437 were from North America; 74,506 were from South America; 174,209 from Europe; 60,269 from Africa, and 5,557 from Oceania. A direct result of the immigration law revisions has been a sharp rise in the influx of Asians (primarily Chinese, Filipinos, Indians, Japanese, and Koreans), of whom 2,738,157 entered the country during 1981–90, as compared with 153,249 during the entire decade of the 1950s. Most immigrants in 2002 came from Mexico (219,380).
Since 1961, the federal government supported and financed the Cuban Refugee Program; in 1995, new accords were agreed to by the two countries. More than 500,000 Cubans were living in southern Florida by 1980, when another 125,000 Cuban refugees arrived; by 1990, 4% of Florida's population was of Cuban descent. Some 169,322 Cubans arrived from 1991–2000, and 27,520 arrived in 2002. Between 1975 and 1978, following the defeat of the US-backed Saigon (Vietnam) government, several hundred thousand Vietnamese refugees came to the United States. Under the Refugee Act of 1980, a ceiling for the number of admissible refugees is set annually; in fiscal 2002, the ceiling for refugees was 70,000. Since Puerto Ricans are American citizens, no special authorization is required for their admission to the continental United States. The population of refugees, asylees, resettled refugees, and asylum-seekers with pending claims was estimated at 5,250,954 in June 2003, a 34% increase over June 2002. During the same year, the newly-formed Bureau of Citizenship and Immigration Services (BCIS—formerly the Immigration and Naturalization Service or INS) received 66,577 applications for asylum, a decline of 36% from 2002. UNHCR reports the United States as the leading destination of refugees, accounting for half of all resettlement worldwide.
Large numbers of aliens—mainly from Latin America, especially Mexico—have illegally established residence in the United States after entering the country as tourists, students, or temporary visitors engaged in work or business. In November 1986, Congress passed a bill allowing illegal aliens who had lived and worked in the United States since 1982 the opportunity to become permanent residents. By the end of fiscal year 1992, 2,650,000 of a potential 2,760,000 eligible for permanent residence under this bill had attained that status. In 1996 the number of illegal alien residents was estimated at 5 million, of which 2 million were believed to be in California. As of 2002, an estimated 33.1 million immigrants (legal and illegal) lived in the United States. Of this total, the Census Bureau estimated in 2000 that 8–9 million of them were illegal alien residents.
The major migratory trends within the United States have been a general westward movement during the 19th century; a long-term movement from farms and other rural settlements to metropolitan areas, which showed signs of reversing in some states during the 1970s; an exodus of southern blacks to the cities of the North and Midwest, especially after World War I; a shift of whites from central cities to surrounding suburbs since World War II; and, also during the post–World War II period, a massive shift from the North and East to the Sunbelt region of the South and Southwest.