Marine animals have provided an excellent source of diverse molecules for drug discovery. In several cases, it is clear that the compounds are not made by the animals themselves but instead by symbiotic bacteria. These bacteria are not randomly distributed, but instead form ancient symbioses with animals, where the interaction centers on small molecule natural products. The resulting patterns of occurrence are directly relevant to the discovery, supply, and engineering of marine natural products. In addition, study of marine symbiosis has uncovered novel enzymatic mechanisms.