Investigation of Plant-Microbe Symbioses (PMS) is the swiftly developing area of modern biology having the strong fundamental and applied impacts. Different types of beneficial PMS (nutritional – N2-fixing and mycorrhizal systems, defensive – endophytic and epiphytic associations) have a lot in common among themselves and to pathogenic systems with respect to molecular, developmental and ecological mechanisms suggesting a necessity to clarify their historical relationships. The purpose of this book is constituted by the evolutionary analysis of PMS aimed at developing the Evolutionary Genetics of Plant-Microbe Symbioses which enables: (i) revealing the general and specific mechanisms for evolution in different PMS types; (ii) using the evolutionary knowledge for genetic improvement and application of PMS in agrosystems.