In recent years, the 1.5 kilos of bacteria that live inside our bodies, mainly in the gut, have proven to be crucially important to our healthy functioning. Beyond their more obvious role in digestion, they are also involved, for example, in the development of the immune system and the neuronal system and in the onset of certain diseases. Three “enterotypes,” or gut types, have been identified that characterize a balanced, yet different relationship between the human host and its personal bacteria. Imbalances of that gut flora have been linked to major health challenges such as diabetes and obesity. Several research teams are now leading cutting-edge projects to further scientific understanding of these relations and, on the longer term, design innovative and tailor-made dietary products, treatments, and prevention tools.