Gut microbiota plays an essential role in the health of our digestive system and has been the subject of extensive research in recent years. These gut microbes have a wide range of roles. Some bacteria provide few or no benefits to the host but are otherwise harmless, e.g., most Escherichia coli living in the gut. Others provide benefits to the host but also benefit from their host. This booklet covers digestive health and why gut health matters and provides an in-depth review of the Human Microbiome Project and what we have learned from it.