U.S. law provides for the temporary admission of foreign nationals. Nonimmigrants are foreign nationals who are admitted for a designated period of time and a specific purpose. There are 24 major nonimmigrant visa categories, which are commonly referred to by the letter and numeral that denote their subsection in the Immigration and Nationality Act (INA); for example, B-2 tourists, E-2 treaty investors, F-1 foreign students, H-1B temporary professional workers, J-1 cultural exchange participants, or S-5 law enforcement witnesses and informants. A U.S. Department of State (DOS) consular officer (at the time of application for a visa) and a Department of Homeland Security (DHS) inspector (at the time of application for admission) must be satisfied that an alien is entitled to 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. Both DOS consular officers (when the alien is applying for nonimmigrant status abroad) and DHS inspectors (when the alien is entering the United States) must also determine that the alien is not ineligible for a visa under the INA's "grounds for inadmissibility," which include criminal, terrorist, and public health grounds for exclusion. In FY2018, DOS consular officers issued 9.0 million nonimmigrant visas, down from a peak of 10.9 million in FY2015. There were approximately 6.8 million tourism and business visas, which comprised more than three-quarters of all nonimmigrant visas issued in FY2018. Other notable groups were temporary workers (924,000, or 10.2%), students (399,000, or 4.4%), and cultural exchange visitors (382,000, or 4.2%). Visas issued to foreign nationals from Asia made up 43% of nonimmigrant visas issued in FY2018, followed by North America (21%), South America (18%), Europe (12%), and Africa (5%). U.S. Customs and Border Protection (CBP) inspectors approved 181.1 million temporary admissions of foreign nationals to the United States during FY2017. CBP data enumerate arrivals, thus counting frequent travelers each time they were admitted to the United States during the fiscal year. Mexican nationals with border crossing cards and Canadian nationals traveling for business or tourist purposes accounted for the vast majority of admissions, representing approximately 103.5 million entries in FY2017. California and Florida were the top two destination states for nonimmigrant visa holders in FY2017, with each state being listed as the destination for more than 10 million nonimmigrant admissions. In addition, nine other states were each listed as the destination for at least 1 million nonimmigrant admissions in that year. Current law and regulations set terms for nonimmigrant lengths of stay in the United States, typically include foreign residency requirements, and often limit what aliens are permitted to do while in the country (e.g., engage in employment or enroll in school). Some observers assert that the law and regulations are not uniformly or rigorously enforced, and the issue of visa overstays has received an increasing amount of attention in recent years. Achieving an optimal balance among policy priorities, such as ensuring national security, facilitating trade and commerce, protecting public health and safety, and fostering international cooperation, remains a challenge.