The United States has long distinguished temporary migration from settlement migration. The Immigration and Nationality Act (INA) provides that foreign nationals may be admitted to the United States temporarily or may come to live permanently. Those admitted on a settlement basis are known as immigrants or legal permanent residents (LPRs), while those admitted on a temporary basis are known as nonimmigrants. Nonimmigrants are admitted for a designated period of time and for a specific purpose. Nonimmigrants include a wide range of people, such as tourists, foreign students, diplomats, temporary agricultural workers, exchange visitors, internationally-known entertainers, foreign media representatives, intracompany business personnel, and crew members on foreign vessels. U.S. immigration policy, embodied in the INA, presumes that all aliens seeking admission to the United States are coming to live permanently. As a result, nonimmigrants must demonstrate that they are coming for a temporary period and for a specific purpose. The U.S. Department of State (DOS) consular officer, at the time of application for a visa, as well as the Department of Homeland Security (DHS) immigration inspectors, at the time of application for admission, must be satisfied that the alien is entitled to a nonimmigrant status. The burden of proof is on the applicant to establish eligibility for nonimmigrant status and the type of nonimmigrant visa for which the application is made. If a nonimmigrant in the United States wishes to change from one nonimmigrant category to another, such as from a tourist visa to a student visa, the foreign national must file a change of status application with the U.S. Citizenship and Immigrant Services (USCIS) in DHS. If the foreign national leaves the United States while the change of status is pending, the foreign national is presumed to have relinquished the application. This report begins with a discussion of the policy tensions surrounding temporary admissions. It follows with a synthesis of the nonimmigrant categories according to the purpose of the visa. It discusses the periods of admission and length of stay and then summarizes grounds for inadmissibility and removal as well as reasons for termination of status. It also describes the circumstances under which nonimmigrants may work in the United States. The second portion of the report analyzes trends in temporary migration. It describes changes over time in nonimmigrant visas issued and nonimmigrant admissions. Various data on nonimmigrants who establish residence in the United States are also discussed. The report concludes with two detailed tables analyzing key admissions requirements across all nonimmigrant visa types. Interest in nonimmigrant visas as a group soared immediately following the September 11, 2001, terrorist attacks, which were conducted by foreign nationals admitted to the United States on temporary visas. At that time, policy makers raised a series of questions about aliens in the United States and the extent to which the federal government monitors their admission and presence in this country. The Enhanced Border Security and Visa Entry Reform Act ( P.L. 107-173 ), provisions in the Homeland Security Act ( P.L. 107-296 ), and provisions in the Intelligence Reform and Terrorism Prevention Act of 2004 ( P.L. 108-458 ), for example, included broad reforms of immigration law to tighten procedures and oversight of aliens temporarily admitted to the United States. Legislative activity, nonetheless, usually focuses on specific visa categories. The temporary worker provisions as well as the foreign student provisions have garnered considerable interest over the years. In addition, legislative revisions to temporary visa categories have usually occurred incrementally. For example, in the 111 th Congress several laws included provisions addressing aspects of temporary admissions, including Division A, Title XVI, §1611 of P.L. 111-5 , which required those employers receiving Troubled Asset Relief Program (TARP) funding to comply with more rigorous labor market rules when recruiting temporary foreign workers, and P.L. 111-230 , which temporarily increased certain fees by $2,000 to $2,250 per nonimmigrant. This report, however, does not track legislation. There is agreement that temporary migration is important to the U.S. economy and cultural life. There is also agreement that temporary migration should be managed more effectively. While revisions to temporary migration do not stoke the same intensity of debate that reform of permanent immigration or mechanisms for legalization do, they do provoke some controversies and concerns. The arguments sketched below are illustrative of the policy tensions surrounding temporary migration generally. In 2009, the United States had a positive $21.9 billion trade surplus in travel and tourism spending because foreign visitors spent more in the United States than U.S. tourists spent abroad. International students contribute nearly $20 billion to the U.S. economy through their expenditures on tuition and living expenses, according to the U.S. Department of Commerce. Almost 70% of all international students' primary funding comes from sources outside of the United States. Expanding temporary worker programs would boost economic output and create new middle class job opportunities for native-born Americans. The user fees paid by foreign nationals on temporary visas do not cover the costs of maintaining the aging infrastructures at ports of entry, nor do they cover the costs for managing the flow of 163 million annual admissions to the United States. The concentration of inspection activity at the border means that significant resources must be present in order to ensure efficient operations. Inefficiencies not only cause congestion, but they can be costly to businesses, both at the border and in the interior. Temporary workers have a deleterious effect on the salaries, compensation, and working conditions of U.S. workers, especially during periods of high unemployment. Tourism and international education and cultural exchange programs foster democratic principles and spread American values across the globe. Foreign visitors to the United States bring energy, ideas, and often fresh perspectives. U.S. employers need the "best and the brightest" workers, regardless of their country of birth, to remain competitive in a worldwide market and to keep their firms in the United States. The ability to hire foreign temporary workers is an essential ingredient for economic growth and is typically based on the human capital needs of the national economy. Appropriately designed temporary worker visas strengthen natural security by re-directing potentially unauthorized migrants into guest workers, relieving the pressure on the border, and enabling DHS Customs and Border Protection (CBP) to focus on terrorists, organized criminals, and violent felons. National security may be put at risk when there is a high volume of nonimmigrants. There is considerable pressure to provide rapid processing of nonimmigrant visas and temporary admissions. Expedited processing, however, can lead to missed opportunities for interdicting threats. It is estimated that each year, hundreds of thousands of foreign nationals overstay their nonimmigrant visas and, as a consequence, become unauthorized aliens. The most recent estimates range from 31% to 57% of the unauthorized population, or approximately 3.3 million to 6.2 million nonimmigrant overstays. DHS Office of Immigration Statistics (OIS) published a report estimating that 1.8 million nonimmigrants had established residence in the United States in 2008. Not only are their ties to the United States tenuous, long-term temporary residents may also weaken support for the common good (e.g., public investments in education, infrastructure, and social programs) among the citizenry. That each of these arguments has some basis in fact is due to the multiplicity of temporary migration categories and their divergent purposes. Positive attributes of one nonimmigrant category may not apply to another. Similarly, critiques of one nonimmigrant category may not hold for another. There are 24 major nonimmigrant visa categories, and 87 specific types of nonimmigrant visas are issued currently. Most of these nonimmigrant visa categories are defined in §101(a)(15) of the INA. These visa categories are commonly referred to by the letter and numeral that denotes their subsection in §101(a)(15); for example, B-2 tourists, E-2 treaty investors, F-1 foreign students, H-1B temporary professional workers, J-1 cultural exchange participants, and S-4 terrorist informants. These temporary visas may be grouped under the broad labels described below. Ambassadors, consuls, and other official representatives of foreign governments (and their immediate family and servants) enter the United States on A visas. Official representatives of international organizations (and their immediate family and servants) are admitted on G visas. Those nonimmigrants entering under the auspices of the North Atlantic Treaty Organization (NATO) have their own visa categories. Aliens who work for foreign media use the I visa. B-1 nonimmigrants are visitors for business and are required to be seeking admission for activities other than purely local employment or hire. The difference between a business visitor and a temporary worker also depends on the source of the alien's salary. To be classified as a visitor for business, an alien must receive his or her salary from abroad and must not receive any remuneration from a U.S. source other than an expense allowance and reimbursement for other expenses incidental to temporary stay. The B-2 visa is granted for temporary visitors for "pleasure," otherwise known as tourists. Tourists, who are encouraged to visit as a boon to the U.S. economy, have consistently been the largest nonimmigrant class of admission to the United States. A B-2 nonimmigrant may not engage in any employment in the United States. Many visitors, however, enter the United States without nonimmigrant visas through the Visa Waiver Program (VWP). This provision of the INA allows the Secretary of Homeland Security to waive the visa documentary requirements for aliens coming as visitors from countries that meet certain statutory criteria. Currently, foreign visitors from 36 countries have VWP privileges.