The United States has exceptionally onerous and complicated visa requirements. Read up carefully before your visit, especially if you need to apply for a visa, and consult the US State Department’s Bureau of Consular Affairs. Travellers have been refused entry for many reasons, often trivial.
There is no airside transit without US entry between international flights. All travellers must disembark and proceed through immigration and customs inspection to enter the United States, even if you're only staying for the two to four hours required to transit between flights. This is most relevant if you're transiting between Asia or Europe to/from Latin America. Therefore, all travellers must be able to enter the United States on the Visa Waiver Program (or other visa exemption) or obtain a visitor's (B1 or B2) or transit (C1) visa.
Law and bureaucracy
The US federal government has five separate agencies with jurisdiction over visitors.
The most important one from a visitor's perspective is Customs and Border Protection (CBP), a bureau of the US Department of Homeland Security (DHS). The CBP's Office of Field Operations operates 20 Field Operations offices which supervise immigration and customs inspection stations at over 320 ports of entry. All travellers entering the United States must undergo immigration and customs inspection to ensure lawful entry. All US citizens and nationals and visitors who can qualify for the Visa Waiver Program (VWP) (as explained further below) generally encounter only CBP officers.
If you cannot enter the US through the VWP, you must visit a US Embassy or Consulate in your home country to apply for and obtain a visa, which will often require a short visa interview with a US consular officer. US Embassies and Consulates are operated by the Bureau of Consular Affairs of the US Department of State.
If you attempt to unlawfully cross a US land border at any other point besides a port of entry, you may encounter the U.S. Border Patrol, which is also part of CBP.
If you attempt to unlawfully come ashore in the US from a body of water at any other point besides a port of entry, you may encounter the U.S. Coast Guard, which is normally part of DHS (but can operate as part of the Department of Defense in wartime).
Finally, if you unlawfully enter the US, commit a severe crime in the US, or overstay your visa, you will likely encounter officers from the division of Enforcement and Removal Operations (ERO) of Immigration and Customs Enforcement (ICE), another DHS bureau. ICE operates a gigantic system of immigration detention facilities. Strict compliance with US law during your stay is strongly recommended. ICE is frequently criticized by human rights organizations like Amnesty International for ongoing problems with substandard healthcare and human rights violations.
Planning and pre-arrival documentation
Citizens of the 38 countries within the Visa Waiver Program, as well as Canadians, Mexicans living on the border (holding a Border Crossing Card), and Bermudians (with British Overseas Territories passports) do not require advance visas for entry into the United States.
For Canadians and Bermudians, the entry period is normally for a maximum of six months. However, entry may still be refused on the basis of a criminal record. Those who have criminal records should seek out a US embassy for advice on whether they need a visa.
For travellers under the Visa Waiver Program, the entry period is strictly limited to 90 days (see additional requirements below).
As of March 2014, the countries under the Visa Waiver Program are Andorra, Austria, Australia, Belgium, Brunei, Chile, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Japan, South Korea, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Monaco, the Netherlands, New Zealand, Norway, Portugal, San Marino, Singapore, Slovakia, Slovenia, Spain, Sweden, Switzerland, Taiwan and the United Kingdom.
Citizens of the Federated States of Micronesia, the Marshall Islands, and Palau may enter, reside, study, and work in the US indefinitely with only a valid passport.
Citizens of the Bahamas may apply for visa-free entry only at the US Customs pre-clearance facilities in the Bahamas, but a valid police certificate may be required for those over the age of 14. Attempting to enter through any other port of entry requires a valid visa.
Persons holding a passport from the Cayman Islands, if they intend to travel directly to the US from there, may obtain a single-entry visa waiver for about $25 prior to departure.