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 archaeological 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 takes place in a brewery by a brewer, and the brewing industry is part of most western economies.

The basic ingredients of beer are water; a starch source, such as malted barley, which is able to be fermented (converted into alcohol); a brewer's yeast to produce the fermentation; and a flavouring, such as hops. A secondary starch source (an adjunct) may be used, such as maize (corn), rice or sugar. Less widely used starch sources include millet, sorghum and cassava root in Africa, potato in Brazil, and agave in Mexico, among others. The amount of each starch source in a beer recipe is collectively called the grain bill.

There are several steps in the brewing process, which include malting, milling, mashing, lautering, boiling, fermenting, conditioning, filtering, and packaging. There are three main fermentation methods, warm, cool and wild or spontaneous. Fermentation may take place in open or closed vessels. There may be a secondary fermentation that can take place in the brewery, in the cask, or in the bottle.