The yeast used in baking is known as Saccharomyces cerevisiae. This species is also used in fermentation of beer and wine.

Yeasts for leavening bread may be produced commercially or caught from the environment. Commercial yeast is prepared by taking one yeast cell, placing it in a test tube, and providing it with food and the moist, warm conditions it needs to thrive. Yeast can reproduce asexually, so it can reproduce itself, creating large numbers of cells.

In bread production, yeast cells convert carbohydrates into carbon dioxide, which causes the dough to expand or rise, and alcohol, most of which evaporates during baking. The use of potatoes, water from potato boiling, eggs, or sugar in a bread dough accelerates the growth of yeasts. Salt and fats such as butter slow yeast growth down.

Baker's yeast comes in two forms. The first form is fresh yeast pressed into a square cake. This form perishes quickly, and must be used soon after production in order to maintain the desired effects. Dry yeast is granulated and has a longer shelf life than fresh yeast. In the production of beer or wine, sugar is converted into alcohol by yeast.

Water can be used to determine if yeast is expired; it will foam if still active.