This report provides a broad overview of U.S. immigration policy. The first section addresses policies governing how foreign nationals enter the United States either to reside permanently or to stay temporarily. Related topics within this section include visa issuance and security, forms of quasi-legal status, and naturalization. The second section discusses enforcement policies both for excluding foreign nationals from admission into the United States, as well as for detaining and removing those who enter the country unlawfully or who enter lawfully but subsequently commit crimes that make them deportable. The section also covers worksite enforcement and immigration fraud. The third section addresses policies for unauthorized aliens residing in the United States. While intended to be comprehensive, this primer may omit some immigration-related topics. It does not discuss policy issues or congressional concerns about specific immigration-related policies and programs. U.S. immigration policy is governed largely by the Immigration and Nationality Act (INA), which was first codified in 1952 and has been amended significantly several times since. Implementation of INA policies is carried out by multiple executive branch agencies. The Department of Homeland Security (DHS) has primary responsibility for immigration functions through several agencies: U.S. Citizenship and Immigration Services (USCIS), Customs and Border Protection (CBP), and Immigration and Customs Enforcement (ICE). The Department of State (DOS) issues visas to foreign nationals overseas, and the Department of Justice (DOJ) operates immigration courts through its Executive Office of Immigration Review (EOIR). Foreign-born populations with different legal statuses are referred to throughout this report. The term a liens refers to people who are not U.S. citizens, including those legally and not legally present. The two basic types of legal aliens are (1) immigrants (not including refugees and asylees) and (2) nonimmigrants. Im mi grant s refers to foreign nationals lawfully admitted to the United States for permanent residence. N onimmigrant s refers to foreign nationals temporarily and lawfully admitted to the United States for a specific purpose and period of time, including tourists, diplomats, students, temporary workers, and exchange visitors, among others. Refugees and asylees refer to persons fleeing their countries because of persecution, or a well-founded fear of persecution, on account of race, religion, nationality, membership in a particular social group, or political opinion (see " Refugees and Asylees "). Refugees and asylees are not classified as immigrants under the INA, but once admitted, they may adjust their status to lawful permanent resident (LPR). Na turalized citizens refers to LPRs who become U.S. citizens through a process known as naturalization (described below), generally after residing in the United States continuously for at least five years. Noncitizen s are persons who have not naturalized and may include immigrants as well as nonimmigrants. U nauthorized alie ns refers to foreign nationals who reside unlawfully in the United States and who either entered the United States illegally ("without inspection") or entered lawfully and temporarily ("with inspection") but subsequently violated the terms of their admission, typically by "overstaying" their visa duration. Four general principles underlie the current system of permanent immigration: family reunification, U.S. labor market contribution, origin-country diversity, and humanitarian assistance. These principles are reflected in different components of permanent immigration. Family reunification occurs primarily through family-sponsored immigration. U.S. labor market contribution occurs through employment-based immigration. Origin-country diversity is addressed through the Diversity Immigrant Visa. Humanitarian assistance occurs primarily through the U.S. refugee and asylee programs. These permanent immigration pathways are discussed further below. The INA limits worldwide permanent immigration to 675,000 persons annually: 480,000 family-sponsored immigrants , made up of family-sponsored immediate relatives of U.S. citizens ("immediate relatives"), and a set of ordered family-sponsored preference immigrants ("preference immigrants"); 140,000 e mployment-based immigrants ; and 55,000 diversity visa immigrants . This worldwide limit, however, is referred to as a "permeable cap" because immediate relatives are exempt from numerical limits placed on family-sponsored immigration (described below) and thereby represent the flexible component of the 675,000 worldwide limit. In addition, the annual number of refugees is determined not by statute but by the President, in consultation with Congress. Consequently, actual total annual LPRs (immigrants, refugees, and asylees) have averaged roughly 1 million persons during the past decade. To ensure that a few countries do not dominate permanent immigration flows, the INA further specifies a "per-country limit" or "cap" limiting the number of family-sponsored preference immigrants and all employment-based immigrants from any single country to 7% of the limit in each preference category. Family-sponsored immigration consists of two immigrant groups ( Table 1 ). I m mediate relative s include spouses and minor unmarried children of U.S. citizens and parents of adult U.S. citizens. An unlimited number of immediate relatives can acquire LPR status each year if they meet the standard eligibility criteria required of all immigrants. Pre ference immigrants , on the other hand, are numerically limited to 226,000 per year and, unlike immediate relatives, are also bounded by the 7% per-country limit. In recent years, family-sponsored immigrants have accounted for two-thirds of all permanent immigration. Employment-based immigration occurs through five numerically limited preference categories ( Table 2 ), the first three of which are ranked by professional accomplishment and ability. The fourth preference category includes various "special immigrants," and the fifth preference category includes immigrant investors (i.e., EB-5 visa holders), a category created in 1990 to benefit the U.S. economy through employment creation and capital investment. Employment-based immigrants include accompanying spouses and children of qualifying LPRs, are limited to 140,000 total annual admissions, and are subject to the same 7% per-country limit as family-sponsored preference immigrants. The number of foreign nationals seeking to immigrate to the United States each year through family-sponsored and employment-based preference categories typically exceeds the INA-mandated numerical limits. Prospective immigrants are further constrained by the 7% per-country cap that primarily impacts foreign nationals from countries that send many immigrants to the United States (e.g., Mexico, China, India, Philippines). As a result, many foreign nationals who meet the U.S. immigrant qualifications and whose petitions have been approved by USCIS must wait years before a numerically limited visa from DOS becomes available. When a visa becomes available, it allows an approved prospective immigrant to travel to the United States and, if admitted to the country by an immigration officer at a port of entry, receive LPR status. If the approved prospective immigrant already resides in the United States on a nonimmigrant visa, the availability of a visa allows him or her to "adjust status" (i.e., change from a temporary nonimmigrant to a permanent immigrant with LPR status) without having to return to the country of origin to complete visa processing through a DOS consulate. As of November 1, 2017, the queue of approved family-sponsored and employment-based immigrants waiting for visas numbered 4.1 million persons. The diversity immigrant visa fosters legal immigration from countries that send relatively few immigrants to the United States. Each year, 50,000 visas are made available to selected natives of countries from which immigrant admissions totaled less than 50,000 over the preceding five years. Since the visa's inception in the early 1990s, the regional distribution of diversity lottery immigrants has shifted from Western European to African and Eastern European countries. To be eligible for a diversity immigrant visa, foreign nationals must have a high school education or two years of work experience within the past five years in an occupation that requires at least two years of training or experience to perform. Diversity immigrant visa applicants are selected by lottery. Winners of the diversity immigrant visa lottery must also meet the standard eligibility criteria required for most immigrants. The United States has long held to the principle that it will not return a foreign national to a country where his or her life or freedom would be threatened. This is embodied in several INA provisions, notably in those defining refugees and asylees. A refugee is a person who is outside his or her home country (a second country that is not the United States) and is unable or unwilling to return because of persecution, or a well-founded fear of persecution, on account of five possible criteria: (1) race, (2) religion, (3) nationality, (4) membership in a particular social group, or (5) political opinion. An asylee is a person who meets the definition of a refugee in terms of persecution or a well-founded fear of persecution but who has been admitted in the United States or is present at a land border or port of entry to the United States. Refugee status is granted within numerical limits. Refugee admissions differ from other immigrant admissions because their annual number, known as the refugee ceiling , and their allocation by world region are not mandated in statute but set each year by the President, in consultation with Congress. For FY2018, the worldwide refugee ceiling was set at 45,000, allocated among world regions (Africa, East Asia, Europe and Central Asia, Latin America/Caribbean, and Near East/South Asia). Asylum is granted on a case-by-case basis and is not numerically limited. An alien in the United States may "affirmatively" apply to USCIS for asylum or may "defensively" seek asylum before an immigration judge during proceedings that determine an individual's removability under the INA.