Microorganisms are essential for plant growth and health. Pathogens cause plant diseases, which can severely reduce yield and quality of our crop plants. On the other hand, other microorganisms are responsible for the turnover of plant residues and nutrients in the soil and thus support the growth of plants. Many microorganisms protect plants against diseases and strengthen their general resistance to adverse conditions, e.g. drought. Knowledge of the biology of microorganisms and their interactions with plants and the soil environment is therefore a prerequisite for the sustainable production of plants that are robust against diseases and environmental stress impacts, including challenges imposed by climate change.