A passport is required for all foreign citizens. Unless eligible under the Visa Waiver Program, foreign travelers must also have a tourist visa.
Travelers entering under the Visa Waiver Program must register with the US government's program, ESTA (https://esta.cbp.dhs.gov), at least three days before arriving; earlier is better, since if denied, travelers must get a visa.
Upon arriving in the US, foreign visitors must register with the Office of Biometric Identity Management, also known as the US-Visit program. This entails having two index fingers scanned and a digital photo taken. For information see www.dhs.gov/obim. Canadian citizens are often exempted from this requirement.
For a complete, up-to-date list of customs regulations, visit the website of US Customs & Border Protection (www.cbp.gov). Each visitor is allowed to bring into the US duty-free 1L of liquor (if you're 21 or older), 200 cigarettes (if you're 18 or older) and up to $100 in gifts and purchases.
Nationals qualifying for the Visa Waiver Program are allowed a 90-day stay without a visa; all others need a visa.
All visitors should reconfirm entry requirements and visa guidelines before arriving. You can get visa information through www.usa.gov, but the US State Dept (www.travel.state.gov) maintains the most comprehensive visa information, with lists of consulates and downloadable application forms. US Citizenship & Immigration Services (www.uscis.gov) mainly serves immigrants, not temporary visitors.
The Visa Waiver Program allows citizens of 38 countries to enter the USA for stays of up to 90 days without first obtaining a US visa (you are not eligible if you are also a national of Iraq, Iran, Syria or Sudan). See the ESTA website (https://esta.cbp.dhs.gov) for a current list. Under this program you must have a nonrefundable return ticket and an 'e-passport' with digital chip. Passports issued/renewed before October 26, 2006, must be machine-readable.
Visitors who don't qualify for the Visa Waiver Program need a visa. Basic requirements are a valid passport, recent photo, travel details and often proof of financial stability. Student visas require extra documentation.
The validity period for a US visitor visa depends on your home country. The length of time you'll be allowed to stay in the USA is determined by US officials at the port of entry. To stay longer than the date stamped on your passport, visit a local USCIS office (www.uscis.gov).
As of 2017, the USA has embarked on a policy of pursuing more stringent border controls. Be warned that the above information is perishable; keep an eye on the news and www.travel.state.gov.