Saccharomyces is a genus of fungi that includes several species of yeasts. These yeasts are commonly found as part of the normal microbiota in the gastrointestinal tract, respiratory system, and vaginal mucosa, where they generally coexist harmlessly. However, they can become pathogenic under certain conditions. Saccharomyces yeasts are unicellular, saprotrophic fungi known for their inability to use nitrate and their ability to ferment a wide variety of carbohydrates.

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Many species of Saccharomyces are critical in the food industry, particularly in the production of fermented products. The most well-known species are Saccharomyces cerevisiae (baker's yeast and brewer's yeast), which plays a central role in the production of bread, beer, and other fermented foods. Brewing strains of Saccharomyces can be classified into two main groups: the ale strains (S. cerevisiae) and the lager strains (S. pastorianus). Lager yeast strains are hybrids of S. cerevisiae and S. eubayanus, and are referred to as "bottom-fermenting" because they ferment at cooler temperatures and settle at the bottom of fermentation vessels. In contrast, ale strains are "top-fermenting," meaning they rise to the top during fermentation in open vessels.

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Saccharomyces cerevisiae has been a cornerstone of human civilization due to its crucial role in food and beverage fermentation, contributing significantly to the commercial production of alcoholic drinks, as well as bread and sourdough. Since the 19th century, S. cerevisiae has been used as a starter culture, originally sourced from the byproducts of beer brewing. Its versatility in fermentation makes it an indispensable microorganism in both the beverage and baking industries.