Plant pathogens employ effector proteins to manipulate their host. During colonization of the plant vasculature, the tomato root- and xylem-infecting fungus Fusarium oxysporum f.sp. lycopersici (Fol) produces fourteen so-called Secreted in Xylem (SIX) proteins. Some of these proteins not only exert a virulence function on a susceptible plant, but also act as avirulence (Avr) proteins in a resistant host as they can be recognized by resistance (R) proteins. Two effector genes, AVR2 (SIX3) and SIX5 form a head-to-head gene pair on the pathogenicity chromosome of Fol strains. The research presented in this thesis describes the virulence-related activities of effectors Avr2 and Six5 of Fol.