In their natural habitats, microorganisms live in permanent exchange with their environment and the surrounding microbes and higher organisms. The nature of these interactions ranges from peaceful symbioses to competition for nutrients and even pathogens infecting hosts. Interactions arising from them are controlled by different chemical signals. Often, small molecules produced by the microorganisms serve as mediators of communication. These low molecular weight substances can serve as attractants, repellents or as virulence factors. Elucidation of microbial communication and of the underlying molecular and biochemical processes is of fundamental importance, for example for the understanding of agriculturally relevant ecosystems in the soil, of host-pathogen interactions during infectious diseases and for the identification of novel pharmaceutically active substances.