Plant resistance is the most important means of controlling diseases and crop losses caused by viruses. This is mainly because other strategies are not available or have not been fully developed. This chapter reviews what is known of natural mechanisms of resistance to plant viruses, questions whether they might be more effectively developed for crop protection, and explores briefly some of the molecular approaches. Resistance mechanisms vary in terms of visual manifestation, effects on virus multiplication and symptom formation, the stage in the viral replicative cycle which serves as the target of the resistance, and in the depth of understanding of the mode of action. In the broadest sense, the replicative cycle of a virus includes not only the biochemical processes leading to the production of progeny particles within the infected host cell, but also processes leading to the dissemination of the virus to other plants, and affecting its persistence in time.