The human intestine harbors up to 10¹¹ bacteria per gram of intestinal content, comprising over 500 different species that have coevolved with their hosts in a mutually beneficial relationship. These bacterial communities promote human health through effects on nutrition and immune system development and function, changing in distinct ways over time and in different disease states. Precisely how bacteria communicate with their hosts to promote immune function is poorly understood. On page 569 of this issue, Smith et al. show that common bacterial metabolites—short-chain fatty acids—selectively expand regulatory T (T_reg) cells in the large intestine. T_reg cells suppress the responses of other immune cells, including those that promote inflammation. This finding provides a new link between bacterial products and a major anti-inflammatory pathway in the gut.