Gueuze is a type of lambic, a Belgian beer. It is made by blending young (1-year-old) and old lambics, which is bottled for a second fermentation. Because the young lambics are not fully fermented, the blended beer contains fermentable sugars, which allow a second fermentation to occur.

Lambic is a type of beer brewed in the Pajottenland region of Belgium southwest of Brussels and in Brussels itself since the 13th century. Types of lambic beer include gueuze, kriek lambic, and framboise. Lambic differs from most other beers in that it is fermented through exposure to wild yeasts and bacteria native to the Zenne valley, as opposed to exposure to carefully cultivated strains of brewer's yeast. This process gives the beer its distinctive flavour: dry, vinous, and cidery, often with a tart aftertaste.