A type of beer brewed traditionally in the Pajottenland region of Belgium (southwest of Brussels) and in Brussels itself. It is now mainly consumed after refermentation, resulting in derived beers such as Gueuze or Kriek lambic.

Unlike conventional ales and lagers, which are fermented by carefully cultivated strains of brewer's yeasts, lambic beer is produced by spontaneous fermentation, being exposed to the wild yeasts and bacteria that are said to be native to the area. It is this unusual process which gives the beer its distinctive flavour: dry, vinous, and cidery, usually with a sour aftertaste.

Today the beer is generally brewed from a grist containing approximately 70% barley malt and 30% unmalted wheat. When the wort has cooled, it is left exposed to the open air so that fermentation may occur. It is then siphoned into old port or sherry barrels and left to ferment and mature for up to 3 years. Another important feature of lambic is that it is usually a blend of at least two different beers.