The requirement for cobalt in biology is predominantly due to the presence of a reactive cobalt center in cobalamin (vitamin B12), a modified tetrapyrrole used as a cofactor by most organisms. Unlike other cofactors, cobalamin is one of nearly 20 biologically active structural variants—corrinoid cofactors known as cobamides—with similar chemical properties. These variants are not functionally equivalent, as different organisms have distinct preferences for different corrinoids. Furthermore, they are produced only by a fraction of prokaryotes, but are used by over 80% of species for diverse metabolic processes such as carbon skeleton rearrangements and biosynthesis of methionine, deoxynucleotides, and natural products. These features together make corrinoids a powerful model for understanding metabolite-sharing interactions in microbiomes. Specifically, because corrinoids play critical roles in microbial metabolism and most microbes rely on corrinoids produced by others, understanding how corrinoids with particular structures are shared among bacteria will lead to a more complete understanding of microbial community interactions. Using biochemical, genetic, bioinformatic, and ecological approaches, we have investigated the mechanisms of corrinoid-based microbial interactions across scales, from the molecular to community levels. At the molecular level, we found that corrinoid preferences in bacteria can be attributed to selectivity in corrinoid-dependent enzymes, corrinoid-responsive regulatory RNA elements (riboswitches), corrinoid uptake, and adenosylation. At the community level, we found that addition of certain corrinoids can transiently shift bacterial composition, even in the complex microbiomes of soil and human gut-derived communities. To determine the molecular basis of these community-level corrinoid effects, we have established bacterial co-cultures, tri-cultures, and higher-order consortia, and found that different microbes have distinct roles in producing and competing for corrinoids. A greater understanding of corrinoid metabolism and structural specificity may lead to improved strategies for manipulating the composition and function of microbiomes