The event will take place in Paisley from 5pm, with more than 5000 fans expected to visit the Town Hall to taste ales from across Europe.
There will be more than 150 Scottish beers as well as 75 from Lincolnshire.
Organised by members of the Renfrewshire branch of the Campaign For Real Ale, the event runs over four days, with 60 breweries providing the beers.
This year, the 27th festival logo celebrates the birth of Paisley poet and writer Robert Tannahill - known as the Weaver Poet - who was born 240 years ago.
The festival celebrates the myriad small Scottish real ale breweries and offers the public the chance to taste lesser known brews.
Derek Moore, festival organiser and founder of the Kelburn Brewery Company in Barrhead, said: "The reason we started the festival was because we felt there was a lack of good beers in the country.
"At that time, in 1986, there were fewer than 20 independent breweries. Now there are about 70 in Scotland.
"We want to let people get a chance to taste the different varieties of real ale that are out there and let them realise it is not always the big companies that make the best beers.
"The atmosphere at the festival is tremendous and it is the friendliest festival I have been to. People from all over the United Kingdom come to it."
Soft drinks and food are also available.
The festival is open 5pm-11pm tonight; noon- 11pm tomorrow; noon-midnight on Friday; and noon-9pm on Saturday. Admission is £6.
For full details see: www.paisleybeerfestival.org.uk