Kvass is a traditional Slavic and Baltic drink, usually made of rye bread, which is called black bread in many Eastern European countries, especially in Ukraine and Russia. The color of the bread used helps to get the color of the drink. According to Russian standards, it is classified as a non-alcoholic beverage because the alcohol content produced by fermentation is usually low (0.5 – 1.0%). It can be flavored with fruits such as strawberries and raisins, or with herbs such as mint.

It is particularly popular in most North Slavic countries, including Poland and Serbia, but not in the Czech Republic or Slovakia. It is also known in the Baltic states of Latvia, Lithuania and Estonia. Kwas is also known in Mongolia, Georgia, Azerbaijan, Tajikistan, Kazakhstan, Uzbekistan and Armenia. Many guvas vendors there sell the drink on the street. Kvass is also popular in Harbin and Xinjiang, China, where Russian culture has had an impact.

Although the introduction of Western soft drinks such as Coca Cola and Pepsi has reduced the commercial sales of Kvas in Russia, Kvas has recently been marketed as a "patriotic" alternative to coke, which has triggered a "resurgence of Kvas". For example, the Russian company Nikolai (whose name, coincidentally, sounds like "not Cola" in Russian) has promoted the brand of Kvas by emphasizing "anti Cola" advertising campaigns. Business analytica in Moscow reported in 2008 that the volume of bottled gas sales has tripled since 2005, and the per capita consumption of gas in Russia is estimated to reach 3 liters in 2008. From 2005 to 2007, coke's share of the soft drink market in Moscow fell from 37% to 32%. Meanwhile, kvass's share more than doubled in the same period, reaching 16% in 2007. In response, Coca Cola launched its own kevas brand in May 2008. This is the first time that foreign companies have entered the Russian gevas market. Pepsi also signed an agreement with a Russian maker of kvass to act as a distribution agent. The development of new technologies for storage and distribution, as well as the emergence of a large number of advertisements, contributed to this popularity. Since 2004, three new major brands have been introduced.