Selected strains of the yeast Saccharomyces cerevisiae cultivated specifically for breadmaking, sold fresh as compressed cakes, dried as granules, or instant for direct mixing. In dough, the yeast ferments sugars into carbon dioxide and ethanol: the gas inflates the gluten network to leaven the bread, while the alcohol and secondary metabolites build flavor. Fresh yeast is perishable and must be refrigerated, active dry yeast needs rehydration in warm water, and instant yeast can be mixed straight into flour, each form suited to different workflows. Beyond leavening, yeast enriches bread nutritionally with B vitamins and contributes the complex aromas of fermentation. It is the same species that brews beer and wine, a single organism domesticated into the foundation of three great fermentation traditions.