This book was originally published in 1985. In the twenty years preceding publication very considerable advances were made in our understanding of the physiology and ecology of lichens and of the functional nature of the symbiosis between the algal and fungal partners. This book provides a critical and selective review of lichen physiological ecology. The approach is a broad one, with consideration given to all the major factors in the lichen's environment and to the important processes of nitrogen fixation, photosynthesis and respiration. The book will be of interest to students and research workers in plant ecology, plant physiology and lichenology.