H. v. Pechman found that coumarin derivatives are formed when malic acid or beta-ketonic esters are condensed with phenols in the presence of concentrated sulfuric acid. This reaction is commonly known s the Pechmann reaction. It has found extensive use. Simonis and co-workers used phosphorus pentaoxide as the condensing agent in place of sulfuric acid and demonstrated that with the same reactants chromones rather than coumarins resulted. The condensation of a phenol and beta-ketonic ester in the presence of phosphorus pentaoxide is sometimes called the Simonis reaction, but it is actually a variation of the Pechmann reaction and is considered in this chapter. Other condensing agents that have been used, for example, are phosphorus oxychloride, phosphoric acid, zinc chloride and others. These are discussed in the chapter. By condensing appropriately substituted phenols and beta-ketonic esters, coumarins can be synthesized with substituents either in the benzene nucleus in the heterocyclic ring or both. The course of this reaction depends on three factors; nature of the phenol, of the beta-ketonic ester, and the condensing agent.