Almost 100 countries do not require visas for U.S. citizens to enter, though they do require entrants to have passports. Countries commonly put limits on the length of stay without a visa. U.S. visitors can stay anywhere from 14 days to six months without having to get a visa, with the majority of countries requiring residence visas after three months.
Most countries in Europe do not require visas for stays of under three months. Great Britain allows Americans to stay six months before requiring a visa. Armenia and Azerbaijan require visas, which are issued on arrival, according to the website Retire-Asia.com. The website also lists which countries do not require visas for entry and those that issue visas on arrival.
Asia and the Pacific
U.S. citizens do not need visas to enter Hong Kong or Macau, but will need a visa if they're planning to continue on to China. Taiwan does not require a visa for visitors arriving at the Taipei airport. Singapore, Thailand, Malaysia, South Korea and the Philippines do not need visas. The Pacific island countries generally do not require visas, but Australia does. Sri Lanka, Laos, Nepal, Papua New Guinea and Tuvalu issue visas on arrival, Retire-Asia.com notes. Western Samoa does not require a visa but issues a visitor's permit on arrival.
Africa and the Middle East
Many African countries issue visas or tourist cards on arrival. Ethiopia and Niger require U.S. citizens to have two passport photos to get the visa on arrival. South Africa, Mauritius, Malawi, Namibia, Israel and Morocco do not require a visa. Lesotho limits visa-free stays to 14 days.
The Americas and the Caribbean
U.S. citizens do not need visas for most North, Central and South American countries. Mexico, Panama and Venezuela require tourist cards. Brazil requires a visa. The Caribbean island countries do not require visas; Cuba is the exception.
Guam, Samoa and Puerto Rico are U.S. territories. A visa is not required for entry.
Visa requirements can change without notice. Always check with the embassy of the country you plan to visit to see if entry requirements have changed. If you're headed for a country that now requires a visa and you don't have one, airline personnel probably will not let you on the plane, unless the country grants visas on arrival.