This review covers the applications of small-molecule and peptidic compounds isolated from marine organisms for biomedical research. Enzymes and proteins from marine sources are already on the market for biomedical applications, but the use of small-molecule biomedical research tools of marine origin is less developed. For many studies involving these molecules the ultimate goal is the application of small-molecule therapeutics in the clinic, but those that do not succeed in the clinic still have clearly defined biological activities, which may be of use as biomedical research tools. In other cases, the investigation of marine-derived compounds has led directly to the discovery of therapeutics with clinical applications. Both as tools and therapeutics, these small-molecule compounds are effective for investigating biological processes, and in this review the authors have chosen to concentrate on the ability of marine natural products to affect membrane processes, ion channels and intracellular processes.