Symbiosis is the fourth volume in the series Cellular Origin and Life in Extreme Habitats (COLE). Fifty experts, from over a dozen countries, review their current studies on different approaches to these phenomena. The chapters present various aspects of symbiosis from gene transfer, morphological features, and biodiversity to individual organisms sharing mutual cellular habitats. The origin of the eukaryotic phase is discussed with emphasis on cyanelles, H syntrophy, N2 fix