Within the body of a healthy adult, microbial cells are estimated to outnumber human cells by a factor of ten to one. The total number of genes in the human microbiome may exceed the total number of human genes by a factor of 100 to 1. These communities are largely unstudied, leaving unknown their influence upon human health. To take advantage of recent technological advances and to develop new ones, the NIH Roadmap initiated the Human Microbiome Project (HMP) with the mission of generating resources enabling characterization of the human microbiota and analysis of its role in human health and disease.