Intercellular communication in plants plays pivotal roles in coordination and control of development and defence responses and involves the trafficking of RNA and protein macromolecules through cytoplasmic cell wall channels termed plasmodesmata. Viruses pirate this existing macromolecular transport pathway to spread infection and, therefore, represent important tools to investigate the cellular mechanisms that govern intercellular communication. The book provides a state of the art overview of the intricate functional virus:host relationships that allow a virus or viroid to move cell-to-cell and systemically through the plant, as well as from plant to plant, and, thus, to spread infection. The book also illustrates the mechanisms by which viruses overcome plant defence responses, such as RNA silencing. Arabidopsis is introduced as a plant host eminently suitable for genetic approaches to identify novel players in plant:virus interactions.