In recent years, marine natural product bioprospecting has yielded a considerable number of drug candidates. Most of these molecules are still in preclinical or early clinical development but some are already on the market, such as cytarabine, or are predicted to be approved soon, such as ET743 (Yondelis). Research into the ecology of marine natural products has shown that many of these compounds function as chemical weapons and have evolved into highly potent inhibitors of physiological processes in the prey, predators or competitors of the marine organisms that use them. Some of the natural products isolated from marine invertebrates have been shown to be, or are suspected to be, of microbial origin and this is now thought to be the case for the majority of such molecules. Marine microorganisms, whose immense genetic and biochemical diversity is only beginning to be appreciated, look likely to become a rich source of novel chemical entities for the discovery of more effective drugs.