Gueuze (or Geuze) is made by blending young (1-year-old) and old (2–3-year-old) lambics into a new beer, which is then bottled for a second fermentation. Because the young lambic is not fully fermented, it contains fermentable sugars, which allow the second fermentation to occur.
Since gueuze is made by blending lambics, it tastes different from traditional ale and lager style beers. Because aged hops are used to produce these lambics, the beer has little to none of the traditional hop flavor and aroma that can be found in most other styles of beer. Furthermore, the wild yeasts that are specific to lambic-style beers give gueuze a dry, cidery, musty, sour, acetic acid, lactic acid taste. In modern times, some brewers have added sugar to their gueuzes to sweeten them and make the beer more appealing to a wider audience. Because of its carbonation, gueuze is sometimes called "Brussels Champagne."
Traditionally, gueuze is served in champagne bottles. The commercial production of gueuze commenced in the 19th century. Both Gueuze and Lambic are protected under Belgian (since 1965) and European (since 1992) law.