Organisers hope to top last year's record attendance of 66,000 over five days and have ordered in another 43,200 pints this time to take the total to more than 340,000.
More than 700 different real ales, ciders and foreign beers are on offer at the Earl's Court exhibition centre in west London.
Brews from South Africa, Japan, Thailand, Australia, Brazil, France, Jamaica, Mexico and Sri Lanka are among those on offer to quench the thirst of adventurous drinkers.
But there was a scare for fans of British beer after a truck heading for Earl's Court packed with 15,000 pints was stolen in central England en route south.
"We knew it was going to be popular, but we've never had the beer stolen before!" said festival organiser Marc Holmes.
"15,000 pints is a lot of beer, but no one will be d