Our lives are each intimately intertwined with the lives of trillions of microbes, those of the human indigenous microbiota. We offer them a unique, regulated, "well-stocked," and protected home; they offer us functional capabilities that dwarf in number and diversity those encoded in our own genome and on which our health depends. Microbial community disturbance is associated with a wide variety, and growing list, of human diseases and disorders. Fundamentally important aspects of the human microbial ecosystem during health and disease have only recently been revealed. These findings include unique features of microbial diversity and community structure, correlations between specific microbial community members or genes and energy harvest from specific dietary components, human individual-specific patterns of microbial diversity and predicted functionality, and indicators of a shared "core microbiome." In this session, we will explore the issue of how well our microbiota resist perturbations, such as antibiotics, and how well they return to their prior state after deliberate perturbation, the role of the human microbiota in host nutritional status, and the contributions of our microbiota to human physiology and metabolic processes.