Dirk Haller and Gabriele Hörmannsperger provide an overview of the central role of the gut and its inhabitants in health, while also giving an initial impression of the complex and dynamic interactions between gut bacteria and the host. Research findings from recent years indicate that the long-overlooked gut bacteria (intestinal microbiota) play an unexpectedly central role in maintaining health. Current research is intensively focused on identifying the foundations for a functioning and health-promoting microbial ecosystem. This knowledge can be used in the future to develop targeted measures for the prevention and treatment of microbiota-associated diseases.