A catalyst is a substance that speeds up a chemical reaction or lowers the temperature or pressure needed to start one, without itself being consumed during the reaction. Catalysts make chemical reactions more efficient by lowering the activation energy, which is the energy barrier that must be surmounted for a chemical reaction to occur. Catalysts can direct a reaction to increase the amount of desired product and reduce the amount of unwanted byproducts, a property called selectivity.

Catalysts can be used in a variety of applications, including the production of biodegradable plastics, new pharmaceuticals, and environmentally safer fuels and fertilizers. Enzymes are naturally occurring catalysts responsible for many essential biochemical reactions. Most solid catalysts are metals or the oxides, sulfides, and halides of metallic elements and of the semimetallic elements boron, aluminum, and silicon. Gaseous and liquid catalysts are commonly used in their pure form or in combination with suitable carriers or solvents; solid catalysts are commonly dispersed in other substances known as catalyst supports.

A catalyst works by providing an alternative pathway for the reaction to occur. During the reaction between the chemical intermediates and the reactants, the catalyst is regenerated. The modes of reactions between the catalysts and the reactants vary widely and in solid catalysts are often complex. Certain solid catalysts, called polyfunctional catalysts, are capable of more than one mode of interaction with the reactants.