The chapter deals with chemical mechanisms that help to control intra- und interspecific interactions with respect to predaceous diving beetles. Apart from chemical receptors and senses within Dytiscidae there are described intraspecific (pheromones) and especially interspecific interactions with respect to this water beetle family. The last group of behavioral modifying compounds includes kairomones and allomones. Allomone constituents from pygidial glands, prothoracic defensive glands, and pupal glands are compiled for a large group of predaceous diving beetles. With respect to the natural compounds, their chemistry, distribution within Hydradephaga, biological activities, and especially their significance for dytiscids are discussed. In addition, further secondary compounds from these beetles are presented, including epicuticular lipids or pigments that may be responsible for the coloration of the adult beetles and their larvae. Finally, the microorganisms and their secondary metabolites that are associated with predaceous diving beetles are presented. The described microorganisms range from culturable to non-culturable
taxa.