The study of the structure and function of tetrapyrrolic compounds has excited the interests of organic chemists, biochemists, botanists and other biologists for more than a hundred years. The structures of most naturally occurring porphyrins were first elucidated by the efforts of Hans Fischer, and great progress was made in our knowledge of chlorophyll by Stoll and Willstatter by applying the best current methods of organic chemistry. The next major advance was made by biochemists who used newly available isotopes of carbon and nitrogen to tackle the formation of porphyrins in biological systems. This was started by the discovery by Shemin and Rittenberg that, of all the amino acids, glycine specifically supplied the nitrogen atoms of protoporphyrin IX. This led to the elucidation of one of the major pathways for the synthesis of 5-aminolaevulinic acid, and ultimately to the origin of all the atoms in protoporphyrin IX. During this period conclusive evidence was also produced that chlorophyll and bacteriochlorophyll are formed in a series of reactions in which protoporphyrin IX is anessential intermediate.