The human microbiome—the collection of microbes living in the gut—is now recognized as an important contributor to health and disease. The environment, the host, and microbe-microbe interactions are all likely to shape the microbiome’s dynamics, but the unique roles of each are not well understood. Now, a Boston Children’s Hospital infectious disease researcher reveals how the microbiome develops in the preterm infant gut, an environment where few microbes are in residence at the beginning of life.