Lambic beer is the name given to beer which is spontaneously fermented.   Instead of having a monoculture of yeast especially bred for brewing beer introduced to the wort, the wort is left open to the atmosphere. Wild yeasts and bacteria falling from the air start to ferment the sugars in the wort and produce their own flavours along with the alcohol and CO2.

The process is not as random as this method suggests as usually the ceiling tiles of the brewery host plenty of micro-cultures which descend like an invisible waterfall into the uncovered fermenters.   As one brewery discovered - change the roof & you change the flavour of the beer.   The old roof was put back underneath the new roof.

The flavours are usually sour and very distinctive from more "regular" beers.   The sourness is often countered with fruit in the beer, most commonly cherries in Kriek.   Raspberries are the next most common and found in Framboise or Frambozen beers depending on which region they are from.

Almost exclusively these beers are brewed in Belgiumⁱ, but the Americans are experimenting with this style too.