The human gut contains several hundred species of microbes and roughly half the total number of cells in our bodies. However, we know almost nothing about these cohabitants or their contribution to human health and disease. Eckburg et al. (p. 1635, published online 14 April 2005) describe the microbial diversity at various sites in the colons of three healthy people by 16S ribosomal RNA-based analysis and offer a comprehensive, detailed molecular survey of the microbiota. The study also suggests analytical and ecological approaches for the future study of complex human endogenous communities.