Brettanomyces is a single-celled fungus that is important in brewing and winemaking as it is resistant to alcohol so can grow even after fermentation starts. It is often shortened to brett.

In wine it is often regarded as a wine fault although in low levels it adds a lot of character to red wines. Some wines, such as Chateau Musar contain high levels, giving them a very distinctive character.

In beer it is more common, and is found for example in Guinness.

Brett is also a very common boy's name.