Many mollusks host symbiotic microbiota that are tightly involved in molluscan biological functions and ecological interactions. Here, we review the described symbioses between molluscan hosts and their bacterial partners. We focus on associations where the molluscan host is hypothesized to gain an evolutionary advantage because of the role of its symbiont. In addition, we focus only on those relationships that have been established experimentally or at least show strong evidence for symbioses. Along with providing a review of the known molluscan host/microbe mutualistic symbioses, we also outline common methodologies in the study of these relationships. Last, we point out areas of further exploration for molluscan microbiome studies.