Yeasts are unicellular fungi that reproduce asexually by budding, a process by which a new cell is formed from a bulge or “bud” that enlarges and pinches off from the parent cell. The nuclear material is divided by mitosis, and the new cell receives a nucleus and cellular organelles before severance from the parent. Yeasts are found in all three of the fungal phyla, but most are ascomycetes. Many are filamentous most of the time, and change to the yeast growth form only occasionally.

Yeasts are of great importance to the baking and brewing industries with particular strains guarded and nurtured closely, because the products of the yeast metabolism give the distinctive flavors to the brews and cause the bread dough to rise in a predictable fashion.