Everybody knows about symbiosis, described by De Bary as the living together of unlike named organisms (1878). True symbiosis benefits both partners, often by simple cross-feeding. Defensive mutualism deepens this definition into associations giving positive defence against environmental hazards, conferred on one or both partners: this includes chemical defence against predators or pathogens, and against environmental extremes. This book explores defensive mutualism across the kingdoms: microbial interactions with plants, animals and fungi, ranging from lichens, mycorrhizae, the marine environment, and secondary metabolites, to the fungus-cultivating ants. It is absorbingly comprehensive and beautifully presented, valuable to all biologists, and a must-have for libraries, teachers and researchers into microbial interactions. These 23 excellent chapters by 56 contributors from nine countries will remain a key resource for many years. Bonuses are excellent colour plates, and reference lists detailing in 129 of the 404 text pages the vast background literature from 1865 to 2008.