Many of the reactions and compounds involved in metabolism are almost iden­ tical in the different groups of living organisms. They are known as primary me­ tabolic reactions and primary metabolic products. In addition, however, a wide variety of biochemical pathways are characteristic of only a few species of organ­ isms, of single "chemical races", or even of a certain stage of differentiation of specialized cells. Such pathways are collectively referred to as "secondary metab­ olism", and the compounds formed are called "secondary products". Secondary products are frequently revealed by their color, smell, or taste. They are responsi­ ble for the flavor of most foodstuffs and beverages and for the color and fragrance of flowers and fruits. Many of them are part of the materia medica, e. g. , alka­ loids, cardiac glycosides, antibiotics, or compounds acting as hormones. Others are used in the industry, e. g. , rubber, tannins, and cellulose. This book treats the organization and significance of biosynthesis, storage, transformation, and degradation of the most important groups of secondary pro­ ducts in microorgimisms, plants, arid animals. It shows that the formation of sec­ ondary products is a common characteristic of specialized cells brought about by the action of special enzymes encoded by specific genetic material.