Trillions of bacteria inhabit the human gut, working in close and complex symbiosis with our cells. Novel analytic methods offer new insights about those complex biochemical interactions, and help us understand how disturbances in their equilibrium can undermine well-being. Researchers are also learning how the gut microbiome responds to the food we eat, influencing obesity, autoimmune disorders, cardiovascular disease, and even mental health. How are diet and lifestyle linked to bacterial communities in the gut? How do we use the growing knowledge about gut health to develop new therapies?