Mechanisms of resistance to plant viruses are diverse, and probably involve different types of recognition events. Often, a cascade of changes affecting broader aspects of defence and metabolism is switched on progressively after the initial recognition event. Virulence, i.e. resistence-breaking behaviour of the virus, involves a failure or alteration of recognition or subsequent signalling. Consequences of these recognition events are the ways in which the pathogenic effects on the host are exerted: formation of visible symptoms and control of plant growth. This volume offers a comprehensive coverage of the recognition and signalling events between plants and viruses whereby the particular attraction of viruses (and viroids) is that they can now be completely defined in molecular terms: they offer excellent opportunities for studying the molecular biology of signalling, and may even provide useful guidelines on how plants and cellular pathogens interact.