It is necessary to realize that the term "photosynthetic prokaryotes" encom- passes the widest and most diverse grouping of bacteria, but in itself has no taxonomic or phylogenetic significance. It means those organisms, other than eukaryotes, which require, obligately or facultatively, light for growth. The re cent application 16S rRNA sequencing to microbial phylogeny, asso- ciated mainly with the work of Woese, emphasizes the evolutionarily dis- persed nature of purpIe and green photosynthetic bacteria as weIl as the rather coherent phylogenetic connections of the cyanobacteria. It is not surprising, therefore, that a volume such as this which seeks to give an introduction to this collection of organisms must be highly selective; ac- cordingly, several important features are discussed only superficiaIly, e. g. , differentiation, life cycles, and biochemical aspects of nutrition. Rather, we have attempted to provide adescription of the essential features which are common and those which are characteristic, e. g. , the physiology of photo- synthesis and ecological distribution. Bearing in mind the aim of the series, we have asked our authors to emphasize aspects of the importance of these organisms in nature and their industrial applications. As will be seen from the text, these organisms have an ancient history in "biotechnology," having been used as foodstuffs by several cultures, but their exploitation has been limited to their natural patterns and products of growth.