Host-parasite interactions have been studied by means of formal genetic procedures since the beginning of this century. It soon proved that resistance of a plant is inherited in a rather simple manner. Parasites, however, have been studied to a much lesser extent than the host plants, largely because of the difficulties in cultivating parasitic organsims and because of their complicated life cycles. Nevertheless, where genetic analysis of both the host plant and the respective parasite has been possible the “gene-for-gene-hypothesis” has been proposed (see Sect. 3.a).