Thisbook examines how the growing knowledge of the huge range of animal-bacterialinteractions, whether in shared ecosystems or intimate symbioses, isfundamentally altering our understanding of animal biology. Individuals fromsimple invertebrates to human are not solitary, homogenous entities but consistof complex communities of many species that likely evolved during a billionyears of coexistence. Defining the individual microbe-host conversations inthese consortia, is a challenging but necessary step on the path tounderstanding the function of the associations as a whole. The hologenometheory of evolution considers the holobiont with its hologenome as a unit ofselection in evolution. This new view may have profound impact on understandinga strictly microbe/symbiont-dependent life style and its evolutionaryconsequences. It may also affect the way how we approach complex environmentaldiseases from corals (coral bleaching) to human (inflammatory bowel diseaseetc). The book is written for scientists as well as medically interestedpersons in the field of immunobiology, microbiology, evolutionary biology,evolutionary medicine and corals.