Brewing is the production of beer through steeping a starch source (commonly cereal grains) in water and then fermenting with yeast. Brewing has taken place since around the 6th millennium BC, and archeological evidence suggests that this technique was used in ancient Egypt. Descriptions of various beer recipes can be found in Sumerian writings, some of the oldest known writing of any sort. Brewing is done in a brewery by a brewer, and the brewing industry is part of most western economies.

There are several brewing methods, such as barrel aging, double dropping, and the Yorkshire Squares and Burton Union systems.

Beers are sometimes aged in barrels to achieve a variety of effects in the final product. Sour beers such as Lambics are aged in wood (usually oak) barrels similar to those used to ferment wine, where a secondary fermentation takes place, usually including microflora other than Saccharomyces cerevisiae. Other beers are aged in barrels which were previously used for maturing spirits. Porters and stouts are sometimes aged in bourbon barrels. Goose Island's Bourbon County Stout was one of the first bourbon barrel-aged beers in the U.S., but the method has now spread to other companies, who have also experimented with aging other styles of beer in bourbon barrels (Allagash Brewing Company, for example, makes a tripel aged in used Jim Beam barrels.)