This book was originally published in 1983. Few groups of organisms have been more successful in forming intimate symbioses with other organisms than the evolutionarily diverse group, the algae. Within every division of these organisms, and in every community they inhabit, symbiotic interactions have evolved, in some cases having profound effects on the ecosystem. Algal symbioses form a continuum, each interaction being a function of the evolutionary history of the separate 'players' as well as the partnership. Each algal symbiosis is unique; although symbioses may be described, they cannot necessarily be categorized. In fact, it is arguable that attempts to do so may obscure the true physiological and genetic nature of the interaction, and bias the scientific objectivity necessary for the required experimental studies of this association. Symbiotic systems provide biologists with tools to study biological phenomena, and are central to understanding cell biology and the origins of innovation in evolution.