Each country has different passport and visa requirements for U. S. citizens. The most commonly visited countries, such as those in Western Europe, generally do not require visas; other countries do require visas, sometimes with specific stipulations. Still other countries require some sort of additional or substitute documentation, usually to ensure that the traveler is merely visiting as a tourist.
Countries with which the United States has troubled relations may require more documentation, from both that country and from the United States. The United States will not allow its citizens to travel to certain countries except for clearly defined business purposes. Because governmental changes in some countries can happen with remarkable speed, it is advisable for travelers to know whether their visit will put them at risk. The Bureau of Consular Affairs at the U. S. State Department keeps an updated list of visa requirements for traveling to every country at its web site http://travel.state.gov/foreignentryreqs.html. Travelers should also contact the foreign embassy or consulate of the country they wish to visit; most often these offices are in Washington, D. C., or New York. Although some countries, such as Canada, Mexico, and certain Caribbean nations, do not require a U. S. passport, it is still a good idea to carry one because of its value as an identification document.