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 asylees, and the diversity of immigrants by country of origin. These principles are embodied in federal law, the Immigration and Nationality Act (INA). The Immigration and Nationality Act Amendments of 1965 replaced the national origins quota system (enacted after World War I) with per-country ceilings, and the statutory provisions regulating legal permanent immigration to the United States were last revised significantly by the Immigration Act of 1990. Despite extensive critiques of the permanent legal immigration system, no consensus exists on the specific direction reforms to the system should take. Some maintain that revising provisions governing legal permanent immigration should be a key component of any major immigration reform proposal, while others support the existing provisions and their underlying rationales. This report on legal permanent immigration may help inform the debate and discussions of policy options that may emerge as Congress considers current immigration proposals. Legal aliens are of three main types: immigrants , nonimmigrants and refugees . As defined in the INA, immigrants are synonymous with lawful permanent residents (LPRs) and refer to foreign nationals who come to live lawfully and permanently in the United States. N onimmigrants —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 must leave the United States before their visas expire, although certain classes of nonimmigrants may adjust to LPR status if they otherwise qualify. Refugees and asylees are people fleeing their countries because of persecution or a well-founded fear of persecution. After one year in refugee status in the United States, refugees must apply to adjust to LPR status. In contrast, asylees may, but are not required to, apply for LPR status after one year. The conditions for the admission of immigrants and refugees are more stringent than for nonimmigrants, and many fewer immigrants than nonimmigrants are admitted each year. 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. Naturalization is voluntary, and persons may remain as LPRs indefinitely as long as they do not commit a crime or some other act that makes them deportable. Prospective immigrants must maneuver a multi-step process through federal departments and agencies to obtain LPR status. First, petitions for LPR status are filed with U.S. Citizenship and Immigration Services (USCIS) in the Department of Homeland Security (DHS) by the prospective immigrant or by the sponsoring relative or employer in the United States (in the case of family-sponsored or employment-based immigration, respectively). If the prospective LPR is residing abroad or has not established a lawful residence in the United States, the petition is forwarded to the Department of State's (DOS) Bureau of Consular Affairs in the alien's home country after USCIS has approved it. If the prospective immigrant is already legally residing in the United States, USCIS handles most of the process, which the INA refers to as "adjustment of status" because the alien is moving from a temporary status to LPR status. Roughly half of all persons granted LPR status in FY2016—the most recent year for which such data are available—did so by adjusting status. The Consular Affairs officer (when the alien is coming from abroad) or the USCIS adjudicator (when the alien is adjusting status from within the United States) must be satisfied that the alien is entitled to LPR status. These reviews are intended to ensure that prospective immigrants are not ineligible for visas or admission under the grounds for inadmissibility spelled out in the INA. Immigrant admissions and adjustments to LPR status are subject to complex numerical limits and preference categories that give priority for admission on the basis of family relationships, needed skills, and geographic diversity. In addition, immigrants who enter through the family-sponsored and employment based preference categories are subject to a 7% per-country cap (see " Per-country Ceilings " below). Numerical limits on immigration combined with the per-country cap for some categories has resulted in a sizable "visa queue" of foreign nationals with approved immigration petitions who must wait until a numerically limited visa becomes available before they can immigrate permanently to the United States (see " The Visa Queue " below). The INA provides for a permanent annual worldwide level of 675,000 LPRs comprising three components: 1. family-sponsored immigrants (480,000 plus certain unused employment-based preference numbers from the prior year), made up of two groups: a. immediate relatives of U.S. citizens and b. family-sponsored preference immigrants; 1. employ ment-based preference immigrants (140,000 plus certain unused family preference numbers from the prior year); and 2. diversity immigrants (55,000). Despite the numerical limits, the annual worldwide level is flexible, and the INA permits certain LPR categories to exceed the limits. For example, although the INA places a limit of 480,000 on family-sponsored immigrants, some refer to that limit as a "permeable cap" because immediate relatives of U.S. citizens are not numerically limited. The annual level of family-sponsored preference immigrants is determined as follows: 480,000 (the total family-sponsored immigration level), minus the number of immediate relatives granted LPR status in the prior year, minus the number of aliens paroled into the United States for at least a year, plus (when available) the number of unused employment preference immigrant from the prior year. As a result, the actual number of immigrants who receive LPR status varies from year to year according to the prior year's number of immediate relative immigrants, parolees, and unused employment-based preference immigrant visas that roll over. Under the INA, the annual level of family preference immigrants may not fall below 226,000. If the number of immediate relatives of U.S. citizens admitted in the previous year happens to fall below 254,000 (the difference between 480,000 for all family-sponsored immigrants and 226,000 for family-sponsored preference immigrants), then family-sponsored preference immigrants may exceed 226,000 by that amount. However, since FY1996, annual admissions of immediate relatives of U.S. citizens have exceeded 254,000 each year, ranging from a low of 258,584 immigrants in FY1999 to a peak of 580,348 immigrants in FY2006; in FY2016 they numbered 566,706. Consequently, the annual limit for family-sponsored preference immigrants has effectively remained at 226,000 for the past two decades. The United States also gives LPR status to persons admitted as refugees and persons who are granted asylum. Refugees and asylees can apply to adjust to LPR status after one year in refugee/asylee status in the United States. Refugees and asylees are exempt from statutory numerical limits. As mentioned above, numerically-limited preference immigrants who enter through the family-sponsored and employment based preference categories are limited by a 7% per-country cap. The per-country level is not a quota or set aside for individual countries, as each of the 195 countries in the world could not receive 7% of the specified limit. As the State Department describes: "(T)he country limitation serves to avoid monopolization of virtually all the annual limitation by applicants from only a few countries. This limitation is not a quota to which any particular country is entitled, however." Congress has enacted two important exceptions to the per-country ceilings. The first exception allows 75% of the visas allocated to the 2 nd family preference category (2A) of spouses and children of LPRs to be exempt from the per-country ceiling (see Table 1 for family preference categories). The second exception allows the 7% per-country ceiling for employment-based immigrants to be surpassed for individual countries that are oversubscribed if visas are available within the 140,000 worldwide limit for employment-based preferences. The impact of these revisions to the per-country ceilings is discussed later in this report. As noted, family-sponsored and employment-based preference category immigrants are numerically limited. Both types of immigrants are subdivided into five categories. Within each family and employment preference category, the INA further allocates the number of people who can receive LPR status each year. The five family preference categories are based broadly upon a hierarchy of family relationships to U.S. citizens and LPRs. Among the five employment preference categories, the first three are based broadly upon a hierarchy of professional accomplishments and skills needed by U.S. employers ( Table 1 ). The fourth category includes 13 sub-categories of "special immigrants," including religious workers, employees of the U.S. government abroad, and juvenile court dependents. As part of the Immigration Act of 1990, Congress added a fifth preference employment-based category that allows foreign investors to acquire LPR status ("EB-5 immigrant investors"). For this preference category, the INA allocates up to 10,000 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 invest in "Targeted Employment Areas" (TEAs), which include rural areas (as defined by the Office of Management and Budget) or areas experiencing unemployment at 150% of the national average. Employers seeking to hire prospective immigrants through the second and third employment-based preference categories must petition the U.S. Department of Labor (DOL) to obtain a labor certification on behalf of the alien. Prospective immigrants must demonstrate that they meet the qualifications for both the particular job and the preference category.