The United States has long distinguished temporary migration from settlement migration. The Immigration and Nationality Act (INA) establishes the circumstances under which foreign nationals may be admitted temporarily or come to live permanently. Those admitted on a permanent basis are known as immigrants, or lawful permanent residents (LPRs), while those admitted on a temporary basis are known as nonimmigrants. The INA provides for the admission of nonimmigrants for designated periods of time and a variety of specific purposes. Nonimmigrants include tourists, foreign students, diplomats, temporary agricultural workers, cultural exchange visitors, internationally known entertainers, foreign media representatives, intracompany business personnel, and crew members on foreign vessels, among others. Policy discussions about nonimmigrant admissions are as varied as the visa classes under which temporary migrants enter. Tourists, business visitors, and foreign students, for example, are usually seen as a boon to the U.S. economy, while the economic costs and benefits of temporary workers are hotly debated. In addition, cultural exchange programs are a foreign policy tool, intended to foster democratic principles and spread American values across the globe, but some of these programs that include work authorization have come under scrutiny for an alleged lack of protections for both U.S. workers and program participants. Further, the entry of nonimmigrants has prompted national security concerns—particularly over those who enter under the Visa Waiver Program. Debates also continue over the implementation of a system to document the exits of foreign nationals from the United States, as nonimmigrants who remain in the country after their visas expire are accounting for a growing share of the unauthorized alien population. Achieving an optimal balance among policy priorities—ensuring national security, facilitating trade and commerce, supporting fair labor practices, protecting public health and safety, and fostering international cooperation—remains a challenge. As policymakers consider modifying nonimmigrant visa categories, they may be interested in learning more about each of the categories and their relationship to the policy priorities. This report explains the statutory and regulatory provisions that govern nonimmigrant admissions to the United States before turning to a description of the major nonimmigrant categories. It describes trends in temporary migration, including changes over time in the number of nonimmigrant visas issued and nonimmigrant admissions. Estimates of nonimmigrants who establish residence in the United States are briefly discussed, as are estimates of those who stay beyond their authorized period of admission. The report concludes with a detailed table showing key admissions requirements across all nonimmigrant visa types. The Department of State (DOS), the Department of Homeland Security (DHS), and the Department of Labor (DOL) each play key roles in administering the law and setting policies on the admission of nonimmigrants. Foreign nationals living outside the United States who wish to enter the country apply for a visa at a U.S. embassy or consulate abroad. Two agencies within DHS handle the admission procedures that occur on U.S. soil: Customs and Border Protection (CBP) determines whether to admit nonimmigrants at U.S. ports of entry (POEs), and U.S. Citizenship and Immigration Services (USCIS) handles applications for extending or changing an individual's nonimmigrant status and the adjudication of visa petitions by employers for nonimmigrant workers. For certain classes of workers, an employer must first submit an application to DOL's Office of Foreign Labor Certification. If approved, the employer petitions USCIS for the visa on behalf of the worker. Finally, DOS interviews the worker and issues a visa if conditions are met. A DOS consular officer (at the time of application for a visa) and a CBP immigration inspector (at the time of application for admission) must be satisfied that an 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. The time period that a visa lasts has two elements: how long the foreign national is authorized to stay in the United States, and how long the visa is valid for entry into the United States. Section 214 of the INA and 8 C.F.R. §214 address length and extensions of stay for the various classes of nonimmigrants. For example, A-1 ambassadors are allowed to remain in the United States for the duration of their service, F-1 students for the duration of their studies, D crew members for up to 29 days, and R-1 religious workers for 30 months (with the possibility of extension for an additional 30 months). H-1B visa holders may stay in the country for three years with the option to renew once for up to six years total. Many categories of nonimmigrants are required to have a residence in their home country that they intend to return to as a stipulation of obtaining a visa. (See the Appendix for requirements for each visa type.) Nonimmigrants who remain in the United States past their lawful period of admission are known as overstays. Separate from the length of stays authorized for nonimmigrant visas is their validity period, or the time during which the visa is valid for travel. These time periods are negotiated country-by-country and category-by-category. For example, a B visitor visa from Germany is valid for 10 years while a B visa from Indonesia is valid for five years. The D crew member visa is valid for five years for Egyptians, but two years for Russians. R-1 visas have a validity period of 36 months for Ugandans and 60 months for Italians. The country-specific validity periods generally reflect reciprocal authorizations of validity periods for U.S. citizens who travel to those countries. DOS publishes frequent updates to reciprocity agreements and explanations of how they affect different visa classes. With the exception of the nonimmigrants who are temporary workers (i.e., H, O, P, R, and Q visa holders), treaty traders (i.e., E visa holders), international representatives (i.e., A and G visa holders), or the executives of multinational corporations (i.e., L visa holders), most nonimmigrants are not allowed to work in the United States. Exceptions to this policy are noted in the Appendix . Working without authorization is a violation of law and results in the termination of nonimmigrant status. Labor market tests are required for employers petitioning for workers under the H visa classes, E-3 professional worker visas for Australians, and D crewmember visas. The H-2 visas require employers to complete the labor certification process, which includes applying to DOL for certification that there are not sufficient U.S. workers who are able, willing, qualified, and available to perform the needed work, and that admitting alien workers will not adversely affect the wages and working conditions of similarly employed U.S. workers. As part of the labor certification process, employers must offer wages at or above specified levels and meet various other requirements. The labor market test required for H-1 and E-3 workers, known as labor attestation, is less stringent than labor certification. Any employer wishing to bring in an H-1B or E-3 nonimmigrant must attest in an application to DOL that the employer will pay the nonimmigrant the greater of the actual compensation paid to other employees in the same job or the prevailing compensation for that occupation; the employer will provide working conditions for the nonimmigrant that do not cause the working conditions of the other employees to be adversely affected; and there is no strike or lockout at the place of employment. The D visa is for crewmembers working on sea vessels or international airlines but typically does not allow the visa holder to perform longshore work. There are a few exemptions. If an employer is requesting authorization for a D visa holder to perform longshore work via one of these exemptions, the employer must attest that the use of alien crewmembers to perform longshore work is the prevailing practice for the activity at that port, there is no strike or lockout at the place of employment, and notice of the attestation has been given to U.S. workers or their representatives. There are no labor market tests for foreign nationals seeking to enter under the other employment-related visas. DOS consular officers (when the alien is applying abroad), CBP inspectors (when the alien is at a United States port of entry), and USCIS adjudicators (when the alien is applying for an extension or adjustment of status) must confirm that the alien is not ineligible for a visa under the "grounds for inadmissibility" found in §212(a) of the INA. These grounds fall into the following categories: health-related grounds, criminal history, 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, permanently ineligible for citizenship, aliens previously removed, and miscellaneous (including polygamists, child abductors, and unlawful voters). The law provides for waivers of these grounds (except for most of the security and terrorist-related grounds) for nonimmigrants on a case-by-case basis. Section 214(b) of the INA generally presumes that all aliens seeking admission to the United States are coming to settle permanently; as a result, most foreign nationals seeking to qualify for nonimmigrant visas must demonstrate that they are not coming to reside permanently. The §214(b) presumption is the most common basis for rejecting nonimmigrant visa applications, accounting for three quarters of ineligibility findings in FY2016. There are three nonimmigrant visas for which "dual intent" is allowed, meaning that the prospective nonimmigrant visa holder may simultaneously seek LPR status.