This is a comprehensive review of literature pertaining to the aquatic fern Azolla and its nitrogen-fixing algal symbiont, Anabaena Azollae. The preceding decade has witnessed an explosive growth in research on Azolla, and hopefully this book will facilitate those efforts. The book is broken into different categories. Azolla has been of traditional interest to botanists and Asian agriculturists because of its symbiotic association with a nitrogen-fixing blue-green alga. Stimulated by the recent energy crisis, the interests of these two groups have merged, resulting in the publication of numerous articles in popular magazines and extension bulletins. These articles have focused on the green manure, nitrogen fixation, and hydrogen production qualities of Azolla by different authors. The intent of this book is to provide a current and comprehensive survey of all available literature on the Azolla symbiosis, essentially up-dating and expanding the excellent reviews of some authors.Thus Azolla is a small multifunctional aquatic plant, with big future, so our interest on this regard has motivated me to work in this field.