Four major principles 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 the country of origin. These principles are embodied in federal law, the Immigration and Nationality Act (INA) first codified in 1952. Congress has significantly amended the INA several times since, most recently by the Enhanced Border Security and Visa Reform Act of 2002 ( P.L. 107-173 ). An alien is "any person not a citizen or national of the United States" and is synonymous with noncitizen . It includes people who are here legally, as well as people who are here in violation of the INA. Noncitizen is generally used to describe all foreign-born persons in the United States who have not become citizens. The two basic types of legal aliens are immigrants and nonimmigrants . Immigrants are persons admitted as legal permanent residents (LPRs) of the United States. Nonimmigrants—such as tourists, foreign students, diplomats, temporary agricultural workers, exchange visitors, or intracompany business personnel—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 5 years. Immigration admissions 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. These include a flexible worldwide cap of 675,000, not including refugees and asylees (discussed below), and a per-country ceiling , which changes yearly. Numbers allocated to the three preference tracks include a 226,000 minimum for family-based, 140,000 for employment-based, and 55,000 for diversity immigrants (i.e., a formula-based visa lottery aimed at countries that have low levels of immigration to the United States). The per country ceilings may be exceeded for employment-based immigrants, but the worldwide limit of 140,000 remains in effect. In addition, the immediate relatives of U.S. citizens (i.e., their spouses and unmarried minor children, and the parents of adult U.S. citizens) are admitted outside of the numerical limits of the per country ceilings and are the "flexible" component of the worldwide cap. The largest number of immigrants is admitted because of family relationship to a U.S. citizen or immigrant. Of the 1,064,318 legal immigrants in FY2001, 64% entered on the basis of family ties. Immediate relatives of U.S. citizens made up the single largest group of immigrants, as Table 1 indicates. Family preference immigrants —the spouses and children of immigrants, the adult children of U.S. citizens, and the siblings of adult U.S. citizens—were the second largest group. Additional major immigrant groups in FY2001 were employment-based preference immigrants , including spouses and children, refugees and asylees adjusting to immigrant status, and diversity immigrants . The Bureau of Citizenship and Immigration Services (BCIS) in the Department of Homeland Security (DHS) is the lead agency for immigrant admissions. Refugee admissions are governed by different criteria and numerical limits than immigrant admissions. Refugee status requires a finding of persecution or a well-founded fear of persecution in situations of "special humanitarian concern" to the United States. The total annual number of refugee admissions and the allocation of these numbers among refugee groups are determined at the start of each fiscal year by the President after consultation with the Congress. Refugees are admitted from abroad. The INA also provides for the granting of asylum on a case-by-case basis to aliens physically present in the United States who meet the statutory definition of "refugee." All aliens must satisfy State Department consular officers abroad and DHS Bureau of Customs and Border Protection inspectors upon entry to the U.S. that they are not ineligible for visas or admission under the so-called "grounds for inadmissibility" of the INA. These criteria categories are: health-related grounds; criminal history; national security and terrorist concerns; public charge (e.g., indigence); seeking to work without proper labor certification; illegal entrants and immigration law violations; lacking proper documents; ineligible for citizenship; and, aliens previously removed. Some provisions may be waived or are not applicable in the case of nonimmigrants, refugees (e.g., public charge), and other aliens. All family-based immigrants entering after December 18, 1997, must have a new binding affidavit of support signed by a U.S. sponsor in order to meet the public charge requirement. The INA also specifies the circumstances and actions that result in aliens being removed from the United States, i.e., deported. The category of criminal grounds has been of special concern in recent years, and the Illegal Immigration Reform and Immigrant Responsibility Act of 1996 expanded and toughened the deportation consequences of criminal convictions. The category of terrorist grounds has also been broadened and tightened up by the USA Patriot Act of 2001. ( P.L. 107-77 ). The annual number of LPRs admitted or adjusted in the United States rose gradually after World War II, as Figure 1 illustrates. However, the annual admissions never again reached the peaks of the early 20 th century. The BCIS data present only 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. 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. 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. There are two major statistical perspectives on trends in immigration. One uses the official BCIS admissions data and the other draws on Bureau of Census population surveys. The BCIS data present only those admitted as LPRs or those adjusting to LPR status. The census data, on the other hand, include all residents in the population counts, and the census asks people whether they were born in the United States or abroad. As a result, the census data also contain long-term temporary (nonimmigrant) residents and unauthorized residents. The percent of the population that is foreign born, depicted in Figure 2 , resembles the trend line of annual admissions data presented in Figure 1 . It indicates the proportion of foreign born residents is not as large as during earlier periods, but is approaching historic levels at the turn of the last century. Figure 2 illustrates that the sheer number—32.5 in 2002—has more than doubled from 14.1 million in 1980 and is at the highest point in U.S. history. Another tradition of immigration policy is to provide immigrants an opportunity to integrate fully into society. Under U.S. immigration law, all LPRs are potential citizens and may become so through a process known as naturalization . To naturalize, aliens must have continuously resided in the United States for 5 years as LPRs (3 years in the case of spouses of U.S. citizens), show that they have good moral character, demonstrate the ability to read, write, speak, and understand English, and pass an examination on U.S. government and history. Applicants pay fees of $310 when they file their materials and have the option of taking a standardized civics test or of having the examiner quiz them on civics as part of their interview. The language requirement is waived for those who are at least 50 years old and have lived in the United States at least 20 years or who are at least 55 years old and have lived in the United States at least 15 years. Special consideration on the civics requirement is to be given to aliens who are over 65 years old and have lived in the United States for at least 20 years. Both the language and civics requirements are waived for those who are unable to comply due to physical or developmental disabilities or mental impairment. Certain requirements are waived for those who have served in the U.S. military. For a variety of reasons, the number of LPRs petitioning to naturalize has increased in the past year but has not reached nearly the highs of the mid-1990s when over a million people sought to naturalize annually, as Figure 3 depicts. The pending caseload for naturalization remains over half a million, and it is not uncommon for some LPRs to wait 1-2 years for their petitions to be processed, depending on the caseload in the region in which the LPR lives. Illegal aliens or unauthorized aliens are those noncitizens who either entered the United States surreptitiously, i.e., entered without inspection (referred to as EWIs), or overstayed the term of their nonimmigrant visas, e.g., tourist or student visas. Many of these aliens have some type of document—either bogus or expired—and may have cases pending with BCIS. The former INS estimated that there were 7.0 million unauthorized aliens in the United States in 2000.