The human microbiota is a complex assemblage of the microbes inhabiting many sites in the human body. Recent advances in technology have enabled deep sequencing and analysis of the members and structures of these communities. Two sites, the vagina and gastrointestinal tract, are highlighted to exemplify how technological advances have enhanced our knowledge of the host-microbiota system. These examples represent low- and high-complexity communities, respectively. In each example, certain community structures are identified that can be extrapolated to larger collections representing multiple individuals and potential disease or health states. One common feature is the unexpected diversity of the microbiota at any of these locations, which poses a challenge for relating the microbiota to health and disease. However, we anticipate microbiota compositional measurements could become standard clinical practice in the future and may become diagnostic for certain diseases or increased susceptibility to certain disorders. The microbiota of a number of disease states are currently being examined to identify potential correlations. In line with these predictions, it is possible that existing conditions may be resolved by altering the microbiota in a positive way.