Humans are host to trillions of microbial colonizers that contribute significantly to human health and disease. Advances in sequencing and other technologies have facilitated dramatic advances in our knowledge of the types and number of organisms colonizing different areas of the body, and while our knowledge of the roles played by the different bacteria, fungi, and archaea has increased dramatically, there remains much to uncover. The microbes that colonize the human gut contribute to vitamin biosynthesis, immune modulation, and the breakdown of otherwise indigestible foods for nutrient harvest. Bacteria and archaea produce various gases as by-products of fermentation, and it is becoming increasingly understood that these gases have both direct and indirect effects on the gut, and may also be used as diagnostic markers, e.g., hydrogen production as measured by breath testing can be used to diagnose bacterial overgrowth. In this article, we review the roles and effects of hydrogen (H2), methane (CH4) and hydrogen sulfide (H2S) in the human gut.