Plant roots may not only be colonized by mycorrhizal fungi, but also by a myriad of bacterial and fungal root endophytes that are usually not considered by the investigators of classic symbioses. This is the first book dedicated to the interactions of non-mycorrhizal microbial endophytes with plant roots. The phenotypes of these interactions can be extremely plastic, depending on environmental factors, nutritional status, genetic disposition and developmental stages of the two partners. The book deals with diversity, life history strategies, interactions, applications in agriculture and forestry, methods for isolation, cultivation, and both conventional and molecular methods for identification and detection of these endophytes. The comprehensive reviews demonstrate the high diversity of interactions and will provoke further studies to better understand the mechanisms which determine whether a plant-microbial interaction remains asymptomatic, leads to disease or to a mutualistic interaction.