Lambic is a very distinctive type of beer brewed traditionally in the Pajottenland region of Belgium (southwest of Brussels) and in Brussels itself at the Cantillon Brewery and museum. Lambic 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: it is exposed to the wild yeasts and bacteria that are said to be native to the Senne valley, in which Brussels lies. It is this unusual process which gives the beer its distinctive flavour: dry, vinous, and cidery, usually with a sour aftertaste.