Kvass is a fermented cereal-based low-alcohol beverage with a slightly cloudy appearance, light-brown colour and sweet-sour taste. It may be flavoured with berries, fruits, herbs, or honey.

Kvass originates from northeastern Europe, where grain production is considered insufficient for beer to become a daily drink. The first written mention of kvass is found in the Primary Chronicle, describing the celebration of Vladimir the Great's baptism in 996. In the traditional method, kvass is made from a mash obtained from rye bread or rye flour and malt soaked in hot water, fermented for about 12 hours with the help of sugar and bread yeast or baker's yeast at room temperature. In industrial methods, kvass is produced from wort concentrate combined with various grain mixtures. It is a popular drink in Russia, Ukraine, Poland, Baltic countries, Finland, and some parts of China.