Pathogens and their natural counterparts interact with one another in a variety of ways and on different levels: molecular, cellular, organismic, and in dynamics of populations. We investigate these levels of interaction to make them useful in terms of a possible application in crop protection. Only a deeper understanding of these interactions enables the full potential of natural antagonistic organisms and natural substances to be used. Therefore, the elucidation of the mode of action of antagonists and natural substances is the basis of every new development of biological control strategies.