Leavening is the process of causing a batter or dough to rise (increase in volume) by adding a leavening agent. Leavening also lightens the texture of baked goods by making the texture more porous. Rising takes place during baking and sometimes before, as with yeast doughs.

Leaveners are either natural or chemical. Examples of natural leaveners are air, steam and yeast. Air beaten into egg whites is the leavening agent for an angel food cake. Steam from the water in the batters of cream puffs and popovers causes the rising that gives these baked goods their volume. Finally yeast produces a gas, carbon dioxide, as it ferments and thus causes doughs to rise.

Chemical leavening agents are primarily baking powder and baking soda. They form carbon dioxide when they come in contact with liquid, causing baked goods, such as butter cakes, brownies and quick breads, to rise.