Catalysis is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst, which is not consumed in the catalyzed reaction and can continue to act repeatedly. Because of this, only very small amounts of catalyst are required to alter the reaction rate in most cases.Catalysis enables the synthesis of new, highly complex small-molecule in a fewer steps, especially enantiomerically pure chiral molecules, with increased atom economy, and often high selectivity and yield. This chapter discusses various reactions such as hydrogenation, carbonylation, carboxylation, hydroformylation, various coupling reactions, protection, de-protection reactions, etc., and their usefulness in API manufacture.