In order to be able to manipulate microorganisms to produce maximally materials of economic importance to humans, but at minimal costs, it is important that the physiology of the organisms be understood as much as is possible. A yeast cell will divide and produce CO2 under aerobic conditions if offered a solution of glucose and ammonium salts. The increase in cell number resulting from the growth and the bubbling of CO2 are only external evidence of a vast number of chemical reactions going on within the cell. The yeast cell on absorbing the glucose has to produce various proteins which will form enzymes necessary to catalyze the various reactions concerned in metabolic pathways with the manufacture of proteins, carbohydrates, lipids, and other components of the cell as well as vitamins which will form coenzymes. This chapter presents relevant elements of the physiology of industrial organisms, the nature of metabolic pathways, primary and secondary metabolites, trophophase-idiophase relationships in the production of secondary products, role of secondary metabolites in the physiology of organisms producing them, pathways for the synthesis of primary and secondary metabolites, carbon pathways for the formation of microbial products derived from primary metabolism and secondary metabolism of industrial importance.