Four major principles currently underlie U.S. policy on legal permanent immigration: the reunification of families, the admission of immigrants with needed skills, the protection of refugees, and the diversity of admissions by country of origin. These principles are embodied in federal law, the Immigration and Nationality Act (INA) first codified in 1952. The Immigration Amendments of 1965 replaced the national origins quota system (enacted after World War I) with per-country ceilings, and the statutory provisions regulating permanent immigration to the United States were last revised significantly by the Immigration Act of 1990. The two basic types of legal aliens are immigrants and nonimmigrants . As defined in the INA, immigrants are synonymous with legal permanent residents (LPRs) and refer to foreign nationals who come to live lawfully and permanently in the United States. The other major class of legal aliens are nonimmigrants—such as tourists, foreign students, diplomats, temporary agricultural workers, exchange visitors, or intracompany business personnel—who are admitted for a specific purpose and a temporary period of time. Nonimmigrants are required to leave the country when their visas expire, though certain classes of nonimmigrants may adjust to LPR status if they otherwise qualify. The conditions for the admission of immigrants are much more stringent than nonimmigrants, and many fewer immigrants than nonimmigrants are admitted. Once admitted, however, immigrants are subject to few restrictions; for example, they may accept and change employment, and may apply for U.S. citizenship through the naturalization process, generally after five years. Petitions for immigrant (i.e., LPR) status are first filed with U.S. Citizenship and Immigration Services (USCIS) in the Department of Homeland Security (DHS) by the sponsoring relative or employer in the United States. If the prospective immigrant is already residing in the United States, the USCIS handles the entire process, which is called "adjustment of status" because the alien is moving from a temporary category to LPR status. If the prospective LPR does not have legal residence in the United States, the petition is forwarded to the Department of State's (DOS) Bureau of Consular Affairs in their home country after USCIS has reviewed it. The Consular Affairs officer (when the alien is coming from abroad) and USCIS adjudicator (when the alien is adjusting status in the United States) must be satisfied that the alien is entitled to the immigrant status. These reviews are intended to ensure that they are not ineligible for visas or admission under the grounds for inadmissibility spelled out in INA. Many LPRs are adjusting status from within the United States rather than receiving visas issued abroad by Consular Affairs. As discussed more fully below, 54.3% of all LPRs adjusted to LPR status in the United States rather than abroad in FY2010. The INA specifies that each year countries are held to a numerical limit of 7% of the worldwide level of U.S. immigrant admissions, known as per-country limits. The actual number of immigrants that may be approved from a given country, however, is not a simple percentage calculation. Immigrant admissions and adjustments to LPR status are subject to a complex set of numerical limits and preference categories that give priority for admission on the basis of family relationships, needed skills, and geographic diversity, as discussed below. The INA provides for a permanent annual worldwide level of 675,000 legal permanent residents (LPRs), but this level is flexible and certain categories of LPRs are permitted to exceed the limits, as described below. The permanent worldwide immigrant level consists of the following components: family-sponsored immigrants, including immediate relatives of U.S. citizens and family-sponsored preference immigrants (480,000 plus certain unused employment-based preference numbers from the prior year); employment-based preference immigrants (140,000 plus certain unused family preference numbers from the prior year); and diversity immigrants (55,000). Immediate relatives of U.S. citizens as well as refugees and asylees who are adjusting status are exempt from direct numerical limits. Figure 1 summarizes these numerical limits governing the permanent worldwide immigrant level. The annual level of family-sponsored preference immigrants is determined by subtracting the number of immediate relative visas issued in the previous year and the number of aliens paroled into the United States for at least a year from 480,000 (the total family-sponsored level) and—when available—adding employment preference immigrant numbers unused during the previous year. By law, the family-sponsored preference level may not fall below 226,000. In recent years, the 480,000 level has been exceeded to maintain the 226,000 floor on family-sponsored preference visas after subtraction of the immediate relative visas. Within each family and employment preference, the INA further allocates the number of LPRs issued visas each year. As Table 1 summarizes the legal immigration preference system, the complexity of the allocations becomes apparent. Note that in most instances unused visa numbers are allowed to roll down to the next preference category. Employers who seek to hire prospective employment-based immigrants through the second and third preference categories also must petition the U.S. Department of Labor (DOL) on behalf of the alien. The prospective immigrant must demonstrate that he or she meets the qualifications for the particular job as well as the preference category. If DOL determines that a labor shortage exists in the occupation for which the petition is filed, labor certification will be issued. If there is not a labor shortage in the given occupation, the employer must submit evidence of extensive recruitment efforts in order to obtain certification. As part of the Immigration Act of 1990, Congress added a fifth preference category for foreign investors to become LPRs. The INA allocates up to10,000 admissions annually and generally requires a minimum $1 million investment and employment of at least 10 U.S. workers. Less capital is required for aliens who participate in the immigrant investor pilot program, in which they invest in targeted regions and existing enterprises that are financially troubled. As stated earlier, the INA establishes per-country levels at 7% of the worldwide level. For a dependent foreign state, the per-country ceiling is 2%. The per-country level is not a "quota" set aside for individual countries, as each country in the world, of course, could not receive 7% of the overall limit. As the State Department describes, the per-country level "is not an entitlement but a barrier against monopolization." Two important exceptions to the per-country ceilings have been enacted in the past decade. Foremost is an exception for certain family-sponsored immigrants. More specifically, the INA states that 75% of the visas allocated to spouses and children of LPRs (2 nd A family preference) are not subject to the per-country ceiling. Prior to FY2001, employment-based preference immigrants were also held to per-country ceilings. The American Competitiveness in the Twenty-First Century Act of 2000 ( P.L. 106-313 ) enabled the per-country ceilings for employment-based immigrants to be surpassed for individual countries that are oversubscribed as long as visas are available within the worldwide limit for employment-based preferences. The impact of these revisions to the per-country ceilings is discussed later in this report. The actual per-country ceiling varies from year to year according to the prior year's immediate relative and parolee admissions and unused visas that roll over. There are several other major categories of legal permanent immigration in addition to the family-sponsored and employment-based preference categories. These classes of LPRs cover a variety of cases, ranging from aliens who win the Diversity Visa Lottery to aliens in removal (i.e., deportation) proceedings granted LPR status by an immigration judge because of exceptional and extremely unusual hardship. Table 2 summarizes these major classes and identifies whether they are numerically limited. Immigration to the United States is not totally determined by shifts in flow that occur as a result of lawmakers revising the allocations. Immigration to the United States plummeted in the middle of the 20 th Century largely as a result of factors brought on by the Great Depression and World War II. There are a variety of "push-pull" factors that drive immigration. Push factors from the immigrant-sending countries include such circumstances as civil wars and political unrest, economic deprivation and limited job opportunities, and catastrophic natural disasters. Pull factors in the United States include such features as strong employment conditions, reunion with family, and quality of life considerations. A corollary factor is the extent that aliens may be able to migrate to other "desirable" countries that offer circumstances and opportunities comparable to the United States. The annual number of LPRs admitted or adjusted in the United States rose gradually after World War II, as Figure 2 illustrates. However, the annual admissions have not reached the peaks of the early 20 th century. The DHS Office of Immigration Statistics (OIS) data present those admitted as LPRs or those adjusting to LPR status. The growth in immigration after 1980 is partly attributable to the total number of admissions under the basic system, consisting of immigrants entering through a preference system as well as immediate relatives of U.S. citizens, that was augmented considerably by legalized aliens. The Immigration Act of 1990 increased the ceiling on employment-based preference immigration, with the provision that unused employment visas would be made available the following year for family preference immigration. In addition, the number of refugees admitted increased from 718,000 in the period 1966-1980 to 1.6 million during the period 1981-1995, after the enactment of the Refugee Act of 1980. Many LPRs are adjusting status from within the United States rather than receiving visas issued abroad by Consular Affairs before they arrive in the United States.