As one of the oldest leavening agents, the yeast used in baking is usually a strain of Saccharomyces cerevisiae. Typically, bread ingredients (flour, water, salt, sugar/honey/syrup, oil/butter) are mixed with yeast and then allowed to sit for a few hours. As the yeast consumes sugar, it gives off carbon dioxide, causing bread to rise.

Yeast can be purchased in several forms:

• Active dry yeast comes as small (typically beige-brown) granules and should be reconstituted in warm (not hot) water for a few minutes prior to mixing.
• Instant yeast has Vitamin C (ascorbic acid) added which causes it to "wake up" faster/easier. It can be added directly to other dry ingredients.
• There are also liquid forms of yeast, particularly as sold to beer brewers. Beer and bread yeast are interchangeable and differing strains of yeast produce subtly different flavors.

If a number of yeast cells are left in a warm solution containing sugars, they will grow and multiply, and can double in population as fast as every 20 minutes.