Originally a British TV show, presented by Chris Tarrant the format was soon bought by TV companies across the globe.

The full game works like this:

Viewers phone a premium rate phone number, to apply to appear on the show. Random applicants are auditioned on the phone, then 10 appear on each show.

The ten contestants are asked a question which involves sorting four items (e.g. sort "Two,Three,Four,Five" into alphabetical order). The contestant who gets the correct answer in the least time takes centre stage for the quiz.

The contestant is asked 15 questions, each of which is a four-option multiple-choice question, one at a time. Each question earns them more money -- it rises (almost) exponentially:

£100
£200
£300
£500
£1,000 *
£2,000
£4,000
£8,000
£16,000
£32,000 *
£64,000
£125,000
£250,000
£500,000
£1,000,000

The contestant may choose not to answer a question at any time, and retire with their winnings so far. If they get a question wrong, they lose everything the won since the last milestone. The milestone questions are the ones marked with a "*". For example, if you had already won £64,000 and then were to answer the £125,000 question wrongly, you would take home £32,000 (the milestone value), losing £32,000.

Along the way, there are likely to be questions to which the contestant won't know the answer. To help them, they have three lifelines, each of which they may use once:

Ask The Audience
The studio audience vote with their idea of the answer, and the contestant is presented with a graph of the results, which they may use to influence their choice of answer.
50/50
Two wrong answers are removed from the selection of four, giving the contestant a 50/50 chance of going on to select the right answer.
Phone a Friend
The contestant may phone one of a selection of friends they chose before the show, and has 30 seconds to ask them the question.
One of the keys to success in Who Wants To Be A Millionaire is strategic use of the lifelines: don't squander them too early, but don't lose everything by failing to use them either.

The big one mil was won by Judith Keppel. Lots of people whinged because it looked as if she already had a bob or two in the bank.