<human>:  What are the visa regulations for people traveling to the United States?
<bot>: The visa regulations for people traveling to the United States depend on their nationality, the purpose of their visit, and the length of their stay. US citizens and permanent residents do not require a visa, but most non-US citizens must obtain a valid visa from an American embassy or consulate prior to entering the US. Some travelers may be eligible for a visa waiver program. Visitors coming to the US for tourist, business, transit, or medical purposes may qualify for a visitor visa. Those coming to the US for study or work may need to obtain a student or work visa. Generally, visitors who stay in the US for more than 90 days must obtain an extension, change their visa status, or leave the country before their visa expires.