Many of the reactions and compounds involved in metabolism are almost identical in the different groups of living organisms. They are known as primary metabolic reactions and primary metabolic products. In addition, however, a wide variety of biochemical pathways are characteristic of only a few species of organisms, of single "chemical races" or even of a certain stage of differentiation of special- ized cells. Such pathways are collectively referred to as "secondary metabolism", and the compounds formed are called "secondary products". Secondary products are frequently revealed by their color, smell, or taste. They are responsible 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. , alkaloids, cardiac glycosides, antibiotics, or compounds acting as hormones. Others are used by 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 products in microorganisms, plants, and animals. It shows that the formation of secondary products is a common characteristic of specialized cells brought about by the action of special enzymes encoded by specific genetic material. It demon- strates that the. biosynthesis of secondary products is typically without signifi- cance for the individual producer cell, but may play a decisive role in the develop* ment and function of the producer organism as a whole.