United States citizens traveling abroad are required to present a United States Passport upon returning to the United States. Passports are required for travelers entering the country by land, sea and air. Citizens without a passport will be denied entry into the country.
The United States has a program with 36 other countries called the Visa Waiver Program (VWP) which allows citizens of these 36 countries to enter the United States for vacation or business purposes without a visa for stays less than 90 days.
Foreign citizens eligible to visit the states under the VWP must have approval from ESTA, the Electronic System for Travel Authorization to enter the United States. The ESTA determines the eligibility of travelers to come into the U.S. Once you have submitted an ESTA, it is good for two years and unlimited visits to the United States.
For countries not involved in the VWP program, a non-immigrant visa is required to enter the United States. Foreign citizens traveling for pleasure or business purposes do not need any other documentation to enter the United States other than their Visa.
There are several different types of visas. Some are for travel and business, others are for journalists and medical treatment purposes. Exchange visitors need different types of visas as well as extra documentation to enter the country. More information about which type of visa you need is available on the U.S. Department of State's website.
