A substance that produces gas in dough or batter, causing it to rise and develop a light, porous texture. Biological leavening agents include yeast, which ferments sugars to release carbon dioxide, and sourdough cultures. Chemical leavening agents, also called raising agents, include baking powder, which releases gas when moistened and heated, and baking soda used with an acid ingredient. Steam and beaten-in air provide physical leavening in products like choux pastry and sponge cakes. The choice of leavening affects flavour, crumb structure, and keeping quality. Accurate measurement matters: too little gives dense products, while too much causes collapse and off-flavours.