The microbial community in each of our bodies influences our physiology, metabolism and immune function, playing roles in both health and disease. This community of bacteria helps its human host digest food, synthesize certain vitamins, and protect against pathogenic bacteria. Recognizing the benefits of “good” microbes begs the question, what does the microbial population look like in a healthy person? And how does the population differ in a person with metabolic or digestive disease? This monograph reviews the role of the human microbiota in achieving and maintaining health across the lifespan.