Yeast are responsible for beer. It is yeast that create alcohol and carbon dioxide as they consume the sugar in yeast. Of course, prior to Louis Pasteur's discovery of yeast fermentation in 1857, they were not known directly to brewers, but for millenia brewers had un-knowingly cultivated yeast living in storage and fermentation barrels in which beer was being fermented. Barrels were often chosen for repeated use when they produced particularly good beer and in this way, brewers were choosing and cultivating the best yeast for beer production. 

The yeast species that is used in beer making is known scientifically as Saccharomyces cerevisiae.

The word "yeast" comes from Old English gist, gyst, and from the Indo-European root yes-, meaning boil, foam, or bubble. Yeast microbes are probably one of the earliest domesticated organisms. People have used yeast for fermentation and baking throughout history. Archaeologists digging in Egyptian ruins found early grinding stones and baking chambers for yeasted bread, as well as drawings of 4,000-year-old bakeries and breweries. In 1680 the Dutch naturalist Antonie van Leeuwenhoek first microscopically observed yeast, but at the time did not consider them to be living organisms but rather globular structures. In 1857 French microbiologist Louis Pasteur proved in the paper "Mémoire sur la fermentation alcoolique" that alcoholic fermentation was conducted by living yeasts and not by a chemical catalyst. Pasteur showed that by bubbling oxygen into the yeast broth, cell growth could be increased, but the fermentation inhibited - an observation later called the Pasteur effect.

Brewers classify yeasts as top-fermenting and bottom-fermenting. This distinction was introduced by the Dane Emil Christian Hansen. "Top-fermenting yeasts" are so called because they form a foam at the top of the wort during fermentation. They can produce higher alcohol concentrations and prefer higher temperatures, typically 16 °C (61 °F) - 24 °C (75 °F), producing fruitier, sweeter, ale-type beers. An example of a top-fermenting yeast is Saccharomyces cerevisiae, known to brewers as ale yeast. "Bottom-fermenting yeasts" are typically used to produce lager-type beers, though can also produce ale-type beers. These yeasts ferment more sugars, leaving a crisper taste, and grow well at low temperatures. An example of bottom fermenting yeast is Saccharomyces pastorianus, formerly known as Saccharomyces carlsbergensis.

For both types, yeast is fully distributed through the beer while it is fermenting, and both equally flocculate (clump together and precipitate to the bottom of the vessel) when it is finished. By no means do all top-fermenting yeasts demonstrate this behaviour, but it features strongly in many English ale yeasts which may also exhibit chain forming (the failure of budded cells to break from the mother cell) which is technically different from true flocculation.
 
In industrial brewing, to ensure purity of strain, a 'clean' sample of the yeast is stored refrigerated in a laboratory. After a certain number of fermentation cycles, a full scale propagation is produced from this laboratory sample. Typically, it is grown up in about three or four stages using sterile brewing wort and oxygen.

Brettanomyces are a genus of "wild" yeast used in brewing lambic beers. There are three main species: Brettanomyces lambicus; Brettanomyces bruxellensis; and Brettanomyces claussenii, which is found in Britain.