Host–microbiota studies in different animals suggest the role of colonized gut microbiota in their health and other physiological processes. The microbiota affects the developmental process, helps to utilize the available resources, and enhances immunity. Caenorhabditis elegans as an invertebrate organism also coevolved with their beneficial microbes, and their colonization is physiologically defined, known as “core microbiota.” Importantly, the conservation of most signaling pathways with other organisms, especially humans, supports the candidature of C. elegans to study host–microbiota interactions. Therefore, the studies on understanding these microbiota-induced pathways may aid to develop interventions towards treating human diseases. In this chapter, we have summarized the information on the composition and function of gut microbiota in C. elegans and the role of their secreted metabolites in affecting the physiology of C. elegans.