The mammalian gut is the richest immune organ in the body and serves as a central location for immune system development, processing, and education. Inflammatory bowel diseases (IBD) provide excellent models for studying both innate and adaptive responses to gut microbes and the host-immune system – microbe interactions in the gut. Microbes are linked to almost all of the known disease-associated genetic polymorphisms in IBD and are critical mediators of environmental effects (through food, hygiene, and infection). Human and animal-based research supports the central role of microbes in IBD pathogenesis at multiple levels. Animal models of IBD only develop in the presence of microbes, and co-housing mice that are genetically susceptible to gut inflammation with normal mice can lead to the development of bowel injury. Recent advances in research technologies, such as deep-sequencing that enables detailed compositional analyses, have revolutionized the study of host–microbe interactions in the gut; however, knowing which bacteria are present in the bowel is likely not sufficient. The function of the microbiota as a community is recognized as a critical factor for gut homeostasis. Animal models of IBD have provided critical insight into basic biology and disease pathogenesis, especially regarding the role of microbes in IBD pathogenesis. Although many of these recent discoveries on host–microbe interactions are not yet applied to patient care, these basic observations will certainly revolutionize patient care in the future. Using such data, we may be able to predict risk of disease, define biological subtypes, establish tools for prevention, and even cure IBD using microbes or their products. A broad spectrum of therapeutic tools spanning from fecal transplantation, probiotics, prebiotics, and microbial products to microbe-tailored diets may supplement current IBD treatments.