Entry requirements for Americans: US citizens require passports.
Entry requirements for UK nationals: UK nationals require a passport valid for duration of stay. Most passport holders can get an Electronic System for Travel Authorisation (ESTA) through the Visa Waiver Programme, which allows travel to the US for up to 90 days. The VWP includes tourism, certain types of business visit and transit to another country.
Entry requirements for Canadians: The most important requirement on entering the U.S. is providing proof of Canadian citizenship. A valid Canadian passport is the best document to prove Canadian citizenship and the right to return to Canada. However, several other documents can serve, depending on the mode of transport. Generally, Canadian citizens do not require visitor, business, transit or other visas to enter the United States from Canada, though there are some exceptions.
Entry requirements for Australians: Passports must be valid for the period of intended stay. If visiting the US for fewer than 90 days, Australian nationals may be eligible to apply for an Electronic System for Travel Authorization (ESTA), and enter under the Visa Waiver Program (VWP).
Entry requirements for South Africans: South Africans must hold a passport valid for duration of stay. A visa is required.
Entry requirements for New Zealanders: Passports must be valid for duration of stay. New Zealand nationals can get an Electronic System for Travel Authorisation (ESTA) under the Visa Waiver Programme for entry into the United States.
Entry requirements for Irish nationals: Passports must be valid for duration of stay. Irish nationals can get an Electronic System for Travel Authorisation (ESTA) under the Visa Waiver Programme for entry into the United States.
Passport/Visa Note: It is highly recommended that passports have at least six months validity remaining after your intended date of departure from your travel destination. Visa is required for short visits unless travelers qualify for entry under the Visa Waiver Program. The Visa Waiver Program (VWP) enables citizens of certain countries to travel to the US for a stay of up to 90 days without a visa. Visitors under the VWP need to register online three days before travel and have an Electronic System for Travel Authorization (ESTA). This allows the US government to screen all visitors before travel. Visitors entering the country under the VWP must have a machine-readable passport (MRP) that has a barcode on the photo page. Travellers under the VWP must have passports that include biometrics if they wish to enter the country without a visa, which means that passports must contain unique personal data such as fingerprints or iris details. All passports must contain a digital photo image in order to travel visa-free. All visitors to the USA have a photograph and two fingerprints taken by an inkless scanner on arrival, including those travelling visa-free under the VWP. As part of the Western Hemisphere Travel Initiative (WHTI), all travellers travelling between the United States and Canada, Mexico, Bermuda, and the Caribbean region are required to present a passport or other valid travel document to enter or re-enter the United States. If departing from the USA, a valid passport is required by immigration authorities. Immigration officials often apply different rules to those stated by travel agents and official sources.
Note: Passport and visa requirements are liable to change at short notice. Travellers are advised to check their entry requirements with their embassy or consulate.