Once considered obscure and largely ignored by microbiologists, the human microbiota has moved centre-stage in biology. The gut microbiota is now a focus of disparate research disciplines, with its contributions to health and disease ready for translation to clinical medicine. The changing composition of the microbiota is linked with changes in human behaviour and the rising prevalence of immunoallergic and metabolic disorders. The microbiota is both a target for drug therapy and a repository for drug discovery. Its secrets promise the realization of personalized medicine and nutrition, and will change and improve conventional dietary management.