Plant pathogens inject so-called effector molecules into the cells of a host plant to promote their growth and reproduction in these hosts. In plant parasitic nematodes, these effector molecules are produced in the salivary glands. The objective of this thesis was to identify and characterize effector molecules produced in the salivary glands of the potato cyst nematode Globodera rostochiensis. A combination of cDNA-AFLP and mining of EST-databases resulted in the identification of a large family of effectors named the SPRYSECs. The SPRYSECs essentially consist of a conserved SPRY domain preceded by a signal peptide for secretion. The SPRYSECs are injected into host cells through the oral stylet. A protein structure model of the SPRYSECs indicated that one particular surface of the proteins in the SPRYSEC family was hypervariable and seemed to undergo diversifying selection. This led us to believe that the SPRYSECs are important players in the co-evolution between plant and nematode. Transgenic potato plants overexpressing SPRYSEC-19 appeared to be two- to five-fold more susceptible to infections of nematodes, the fungus Verticillium dahliae, and tomato spotted wilt virus (TSWV). This hypersusceptibility to a range of unrelated plant-pathogens suggests that SPRYSECs somehow suppress the basal defense responses that are controlled by the plant’s innate immunity. SPRYSEC-19 was found to engage in a specific physical interaction with the Leucine Rich Repeat domain of a protein from the CC-NBS-LRR class of resistance genes. Many immune receptors in the plant’s innate immunity belong to the same class of NB-LRR proteins. The host interactor of SPRYSEC19 is most similar to members of SW5 R gene cluster that confers resistance to tospoviruses. Remarkably, plants harboring the CC-NB-LRR interactor of SPRYSEC19 are not resistant to nematodes. Therefore, we hypothesize that the nematode effector SPRYSEC-19 promotes its virulence in susceptible host plants by suppressing basal defense through its interaction with an NB-LRR immune receptor.