Growing interest in probiotics, research advances in fecal transplantation, and rising infection rates from bacteria such as C. difficile have brought the gut microbiome to the fore of public discussions and personal decisions about nutrition and health. Trillions of bacteria, fungi, and other microorganisms inhabit our bodies—together influencing vital processes such as digestion and offering potential new avenues to treat a range of persistent health problems. SciLine’s August 22 briefing described the role of gut bacteria in the body, the gut microbiome’s relationship to obesity and other common diseases, and the state of science in fecal microbiota transplantation as a treatment for C. difficile infections.