Fermented tea (also known as post-fermented tea or dark tea) is a class of tea that has undergone microbial fermentation, from several months to many years. The exposure of the tea leaves to humidity and oxygen during the process also causes endo-oxidation (derived from the tea-leaf enzymes themselves) and exo-oxidation (which is microbially catalysed). The tea leaves and the liquor made from them become darker with oxidation. Thus, the various kinds of fermented teas produced across China are also referred to as dark tea, not be confused with black tea. The most famous fermented tea is Pu-erh, produced in Yunnan Province,  and the Anhua black tea produced in Anhua County of Hunan Province.
The fermentation of tea leaves alters their chemistry, affecting the organoleptic qualities of the tea made from them. Fermentation affects the smell of the tea and typically mellows its taste, reducing astringency and bitterness while improving mouthfeel and aftertaste. The microbes may also produce metabolites with health benefits.
The fermentation is carried out primarily by molds. Aspergillus niger was implicated as the main microbial organism in the Pu-erh process, but that species identification has been challenged by comprehensive PCR-DGGE analysis, which points to Aspergillus luchuensis as the primary agent of fermentation.
Most fermented teas are made in China, but several varieties are produced in Japan. In Shan State, Myanmar, lahpet is a form of fermented tea that is eaten, and similar pickled teas are also eaten in northern Thailand and southern Yunnan.