The ultimate goal of the Collaborative Research Center (CRC) 1182 “Origin and Function of Metaorganisms” (www.metaorganism-research.com) at Kiel University is to understand why and how microbial communities form long-term associations with hosts from diverse taxonomic groups. We are particularly interested in the specific functional consequences of the interactions, the underlying regulatory principles, and the resulting impact on host life history and evolutionary fitness in selected host systems.