AT A remote frontier in eastern Turkey, a man is doing a roaring trade in English pounds 5 notes. At other Turkish frontier posts, visitors can pay the pounds 5 visa fee in any currency, or use a pounds 10 note to pay for two people. At this border, however, the rule is that only a fiver is acceptable; nothing else will do. Travellers are aghast - will they have to take the road back to Damascus for currency conversion? Most are relieved to learn that this man will happily sell them a fiver, until they hear the price: dollars 20, about pounds 12.

Getting a passport is only a start if you want to travel the world these days. Whether you are heading for Ankara or Adelaide, you need a visa. Anyone aiming for deepest Africa or Asia will find their passport fills up alarmingly quickly with a variety of exotic (and often expensive) stamps. And the rules change maddeningly frequently.

The most foolproof way to ascertain whether or not your destination requires a visa is to call the embassy. Be warned, though, that even when you and the embassy agree about the visa rules, other parties may not. In July, Edward Pearson and Alexandra Robah set off to visit Pakistan, having been assured by the embassy in London that no visa was necessary for visits of up to 30 days. At Heathrow, however, the check-in staff denied them boarding. The next day the Pakistani embassy confirmed no visa was necessary, but gave them the appropriate stamps anyway to avoid any further grief.

Even when everyone agrees you have the right paperwork, a visa is no guarantee that you will be allowed in; it merely confers the right to apply for entry to a country. It often needs to be supported by a ticket out of the country, evidence of sufficient funds and a vaccination certificate.

While you may know the rules, do not assume that individual immigration officials will share your understanding. The writer Emily Hatchwell spent a day traversing El Salvador to the frontier with Honduras, where the chief immigration officer read her passport with an enthusiasm normally reserved for racy novels. Then he came across an Egyptian entry stamp, which - he deemed - made her unsuitable. 'I wouldn't have minded,' she said, 'except there was a civil war on and I had to make my way back across the country to San Salvador airport.' She bought a plane ticket to the Honduran capital, Tegucigalpa, where officials found her visa perfectly satisfactory.

Consular departments of embassies (high commissions for Commonwealth countries) issue visas. This is usually best done in your home country, but most visas have a limited shelf-life. People planning a long trip may find themselves having to break their journey to spend a day or two incarcerated in consulates in a corner of some foreign capital.

Whether you try in London or Lusaka, applying for a visa can be as complicated as ordering breakfast in an American diner. Would that be a transit, tourist or business visa? Single, double or multiple entry? Will you be arriving by land or air? Travelling independently or on a package tour? The application form will probably contain personal questions about your family, ranging from your mother's maiden name to any criminal convictions (yours, not your mother's). You may also have to demonstrate financial respectability. In order to get a visa for Guyana, I had a gruelling interview, and half way through I had to fetch a bank statement to back up my claim to be solvent.

To obtain any visa requires time; many consulates also demand a hefty fee. And even if you show up in person at the Tanzanian High Commission in London, the rules insist that payment must be in postal orders.

But the United States needs no lessons in rule-making. From next week you must write to the US Embassy to get a visa application form and payment slip. Then you go to Barclays Bank, clutching pounds 13.75 in cash. Once the payment slip has been stamped you are left with two halves. You detach one and send it with application form and passport to the embassy. You are supposed to keep the other half for your records, or just a memento of pounds 13.75 - because even if your application fails, you get no refund.

Given the bureaucratic complexity that a trip can entail, many travellers hand over the problem to a visa agency. They make a charge for their services in addition to the fee charged by the consulate, but you may find the saving in time and legwork worthwhile. Thomas Cook and Trailfinders offer visa services through their branches, and there are several specialist agencies.

Thames Consular Services, 363 Chiswick High Road, London W4 (081-995 2492). The Visa Service, 2 Northdown Street, London N1 (071-833 2709). Worldwide Visas, 9 Adelaide Street, London WC2 (071-379 0419).