Filamentous plant pathogens are threads for global food security, shown in its extremes by an outbreak of Phytophthora infestans leading to the great famine in Ireland. In the addition to classic effector proteins, small RNAs (sRNAs) emerged as another class of effectors to modulate the interaction of plants and pathogens. The underlying mechanism termed cross-kingdom RNA interference (ck-RNAi) was first reported for the necrotrophic fungus Botrytis cinerea (Weiberg et al., Science, 2013). Recent studies further revealed ck-RNAi to contribute to the virulence of the oomycete Hyaloperonospora arabidopsidis (Dunker et al., 2020). This obligate biotrophic oomycete covers a narrow host range including Arabidospsis thaliana, and is the causing agent of the downy mildew disease. However, our current understanding of ck-RNAi is still limited as a consequence of the proof of principal approaches in previous studies. In my current project, I use whole transcriptome analysis coupled to other next generation sequencing approaches to generate a broader picture of the underlying mechanisms involved in ck-RNAi for the H. arabidopsidis - A. thaliana pathosystem. This knowledge could be beneficial for the usage of sustainable sRNA-based plant protection strategies.