B. anomalus B. bruxellensis B. claussenii B. custersianus B. naardenensis B. nanus Brettanomyces is a non-spore forming genus of yeast in the family Saccharomycetaceae, and is often colloquially referred to as "Brett".

The yeast is acidogenic, and when grown on glucose rich media under aerobic conditions, produces large amounts of acetic acid.

It is thought Brettanomyces can be introduced to a winery by insect vectors such as fruit flies, or by purchasing Brett-contaminated wine barrels.

The ability to metabolise the disaccharide cellobiose, along with the irregular surface of a barrel interior, provide ideal conditions for Brettanomyces growth.

Gueuze and other lambic beers owe their unique flavour profiles to Brettanomyces, as do wild yeast saison or farmhouse styles; and it is also found in Oud Bruin and Flanders red ale.

Some American brewers that use Brettanomyces may also include lactic acid producing bacteria such as Lactobacillus and Pediococcus in order to provide sourness to the beer.

While Brett is sometimes pitched into the fermenter, aging in wood barrels previously inoculated with Brettanomyces is another method used to impart the complexity contributed by these strains of yeast.