A multiplicity of biotrophic micro-organisms interact with plants in nature, forming symbiotic relationships that range from mutualism to antagonism. Microorganisms that have adopted biotrophy as a lifestyle are able to colonize the plant and often to cross the plant cell boundaries by forming intracellular structures that are the site of nutrient uptake/exchange. To establish themselves within plant tissues, both mutualistic and pathogenic biotrophs need to overcome the plant defense response through an exchange of molecular signals. Our knowledge of the nature of these signals and their function in the interaction has rapidly increased over the last few years. This volume focuses on the genetic, molecular and cellular components involved in the communication between partners of well-known symbioses, but also reports on the advances for less studied systems.