Host-microbe interactions can be positive or negative for both a host organism and its symbiotic microbes. Interacting with microbes can allow eukaryotes to acquire crucial functions, like nutrient production, but interactions can also be pathogenic in nature. This cross-disciplinary meeting will bring together scientists using cellular, molecular, ecological, physiological, and theory-based approaches to explore the evolution of intimate species-species interactions. Our aim is to understand the cellular systems and pathways that drive transitions in symbiotic states, for example, as partnerships move from pathogenesis to beneficial and back again.