Baking powder is a dry chemical leavening agent used in baking. There are several formulations; all contain an alkali, typically sodium bicarbonate (baking soda), and an acid in the form of salt crystals, together with starch to keep it dry. When dissolved in water the acid and alkali react and evolve carbon dioxide gas, which expands existing bubbles to leaven the mixture. Most modern baking powders contain two acid salts, one which reacts at room temperature, producing a rise as soon as the dough or batter is prepared, and another which reacts at a higher temperatures, causing a further rise during baking. Most recipes call for a mixing procedure that is designed to introduce many tiny air bubbles, for example, "cream the butter and sugar", which the leavening gas from baking powder will expand.

Common low-temperature acid salts include cream of tartar, calcium phosphate, and citrate. High-temperature acid salts are usually aluminium salts, such as calcium aluminum phosphate . They can be found not only in many baking powders, but also in many non-dairy coffee creamers. While dietary aluminium is not known definitively to be detrimental to human health, baking powders are available without it for people who are concerned, and also for those sensitive to the taste.

While various baking powders were sold in the first half of the 19th century, our modern variants were discovered by Alfred Bird who patented Bird's Custard in Birmingham, England. Eben Norton Horsford , a student of Justus von Liebig, who began his studies on baking powder in 1856. Rudolf Oetker , a German pharmacist, made baking powder very popular when he began selling his mixture to housewives. The same recipe he created in 1893 is still sold as Backin in Germany. Oetker started the mass production of baking powder in 1898 and patented his technique in 1903.