True story: An American flew to Brazil and got off the plane, but was soon on the next flight back to the United States, having never even sipped a caipirinha.

What happened? No visa. Serious oops.

While the passport serves as an ID card and proof of citizenship, the tourist visa is the permission slip needed to enter some countries. Without it, the door stays closed. You can knock all you want, but you're not getting in.

As a State Department official puts it: "No one is allowed to enter a country that they're not a citizen of" without permission from that country. "It's a privilege."

Visas allow countries to control the parade of incoming guests as well as weed out any safety threats, such as terrorists or potential disease-carriers.

For most U.S. citizens, however, the visa process is a mere formality, like getting your car inspection sticker. "Americans can travel to a lot of countries without a visa," the State Department official told me, "mainly because they aren't a risk for illegal immigration."

In dozens of foreign destinations, it's good to be an American. Numerous countries waive the visa requirement as part of a reciprocal agreement or because of our trusted standing in the world.

For example, we don't need visas for the 36 countries that are part of the Visa Waiver Program (nor do their citizens need visas to visit the United States). Many Western, Central and Eastern European countries appear on the list, including the United Kingdom, Greece, France, Denmark, Spain, Italy, the Czech Republic, Slovenia and Latvia, to name a few. Some Asian nations (Japan, South Korea, Brunei) and lands Down Under (New Zealand, Australia) are also part of the arrangement.

A batch of countries not affiliated with the VWP also waive the requirement, with restrictions -- mainly limiting the time you can spend in the country, for instance, to 30, 90 or 120 days. Among these are Ukraine; Canada; the Caribbean and most of Central and South America (minus Brazil and Paraguay); the South Pacific islands of Fiji, Tonga and Vanuatu; and a trickle of African countries, including Botswana, South Africa, Senegal and Namibia.

If you disdain extra paperwork and additional fees (from double to triple digits), you can easily spend a lifetime toodling around visa-free countries. But that plan means no pushpin for China, Russia, Nepal, Indonesia or Jordan, among others.

How to get started

To determine whether you need a visa, start with the State Department's Country Specific Information, found online at www.travel.state.gov/travel. The fact-packed descriptions include entry and exit details. In general, the process is similar for every destination. You'll need a valid passport, a completed application and a form of payment (warning: not all consulates take credit cards), plus some extra time to submit the documents and retrieve them from the consulate (in person or by mail). For more details, such as fees and required documents, check the website of the consulate or embassy, and call the office to confirm the information.

A few countries, such as Turkey, Qatar and Kenya, allow travelers to procure a visa upon arrival at the airport or at border checkpoints. But Medhy Habimana, director of operations at VisaHQ (www.visahq.com), which assists travelers with the visa process, doesn't recommend waiting till the plane lands. Apply in advance, "because you'll want to avoid the lines at the airport," he said, "and there may be confusion with the language." Also, by taking care of business before you depart, you won't risk such serious blunders as forgetting to pack passport-size photos -- an oversight that could bar you from entering the country.

When writing out your vacation to-do list, put visa application at the top. The process can take from a few days to several weeks. Habimana says that China typically takes four days, speedy compared with Russia's two weeks during peak season. According to Travisa Outsourcing, which handles India's visas, the waiting time is up to 48 hours for in-person applications and seven to nine business days by mail. Travelers squeezed for time can often pay a supplemental fee for expedited service. For example, China charges $20 more for express service (two to three business days) and $30 for same-day rush service.

Give yourself plenty of time

To avoid meltdowns and fraught exchanges with consulate employees, do your research and be organized. Go down the checklist of required documents, starting with your passport. It should be valid for at least six months after your departure date and should include several blank visa pages.

Some countries, such as Vietnam, keep it simple, requiring a passport, a 2-by-2-inch photo, a completed application form and payment. Others, such as Indonesia, are more demanding: You'll need to present a round-trip itinerary, a letter of employment and proof of sufficient funds from your bank. "There's a lot of stuff the embassies require," said Habimana. "Without providing it, you won't get a visa."

In addition, before leaving the house on your visa quest, check the office hours, which can be highly restricted. The Brazilian consulate, for instance, is open only from 9 a.m. to 1 p.m. weekdays. Foreign offices also close for holidays -- theirs and ours.

If you're short on time or overwhelmed by the procedure, consider hiring a third party to shepherd your application through the system. Rates at VisaHQ, which has offices in Washington, D.C., and New York City, start at $44.95, plus embassy fees. For similar companies, search online for "visa services."

"If your visa is denied," said Habimana, "they don't give an explanation."

Do it right, though, and you probably won't become a visa casualty.