The oomycete Phytophthora infestans is the causal agent of potato late blight. This pathogen exploits a variable repertoire of effector proteins for manipulating plant defence and facilitating colonization. Resistance against the late blight pathogen is largely conferred by a group of plant resistance (R) proteins that each recognize a specific effector secreted by the pathogen thereby activating immunity. This thesis focuses on one of these avirulence effectors named AVR1, the counterpart of potato R1. AVR1 belongs to the class of RXLR effectors that are translocated to the inside of the host cell. For unravelling the molecular processes underlying effector-modulated susceptibility and effector-triggered immunity, the subcellular localization required for AVR1-R1 recognition was investigated and Sec5, a host protein important for intracellular vesicle transport and targeted by AVR1, was functionally analysed. The knowledge gained in this study may help in designing sustainable and durable strategies to control late blight.