Phytophthora species are notorious phytopathogens that reduce crop yield and also affect our ecosystems. To obtain specific and sustainable solutions for plant protection, we need a better understanding of the molecular interactions between plants and pathogens. Intracellular organelles such as chloroplasts and mitochondria play a central role in plant immunity and it is becoming clear that they are important targets of pathogens. Increasing evidence suggests that pathogens and especially oomycetes secrete effector proteins that directly interfere with organellar functions. Using a state-of-the-art organellar proteomic approach, this project aims to identify novel organelle-targeting Phytophthora effectors. Characterization of their mode of action will provide novel insights into how pathogens counter attack the plant immune system and/or redirect cellular metabolism and physiology to their own benefit. Thus far, effectors have mostly been studied in relation to targeting positive regulators of the plant immune system, yet effectors can also target “susceptibility factors” which promote pathogen infection. Susceptibility genes are interesting targets for plant breeding or engineering since they have been shown to provide broad-spectrum and durable resistance. Identifying novel virulence effectors and their plant targets, including potential susceptibility factors, will provide novel leads towards breeding resistant cultivars and/or targeted pest management solutions.