Thailand Guide
Entry requirements
There are three main entry categories for visitors to Thailand; for all of them your passport must be valid for at least six months from the date of entry. As visa requirements are often subject to change, you should always check before departure with a Thai embassy or consulate, a reliable travel agent, or on the Thai Ministry of Foreign Affairs' website at 
www.mfa.go.th/web/2637.php . For further, unofficial details on related matters, such as the perils of overstaying your visa, go to www.thaivisa.com .
Most Western passport holders (that includes citizens of the UK, Ireland, the US, Canada, Australia, New Zealand and South Africa) are allowed to enter the country for short stays without having to apply for a visa (officially termed the "tourist visa exemption"). You'll be granted a thirty-day stay at an international airport but, in a recent change, only fifteen days at an overland border; the period of stay will be stamped into your passport by immigration officials upon entry. You're supposed to be able to somehow show proof of means of living while in the country (B10,000/person, B20,000/family), and in theory you may be put back on the next plane without it or sent back to get a sixty-day tourist visa from the nearest Thai embassy, but this is unheard of.
If you're fairly certain you may want to stay longer than fifteen/thirty days, then from the outset you should apply for a sixty-day tourist visa from a Thai embassy or consulate, accompanying your application – which generally takes several days to process – with your passport and two photos. The sixty-day visa currently costs, for example, US$35 or £28 in the UK; multiple-entry versions are available, costing US$35 or £28 per entry, which may be handy if you're going to be leaving and re-entering Thailand. Ordinary tourist visas are valid for three months, ie you must enter Thailand within three months of the visa being issued by the Thai embassy or consulate. Visa application forms can be downloaded from, for example, www.mfa.go.th/web/2637.php .
Thai embassies also consider applications for ninety-day non-immigrant visas (£45 or US$65, for example, single entry, £100 or US$175 multiple-entry) as long as you can offer a good reason for your visit, such as study or business (there are different categories of non-immigrant visa for which different levels of proof are needed). As it's quite a hassle to organize a ninety-day visa from outside the country (and generally not feasible for most tourists), it's generally easier, though more expensive, to apply for a thirty-day extension to your sixty-day visa once inside Thai borders.
It's not a good idea to overstay your visa limits. Once you're at the airport or the border, you'll have to pay a fine of B500/day before you can leave Thailand. More importantly, however, if you're in the country with an expired visa and you get involved with police or immigration officials for any reason, however trivial, they are obliged to take you to court, possibly imprison you, and deport you.