The marine environment has proven to be a rich source of both biological and chemical diversity and has, therefore, become the focus of a major research effort in natural products drug discovery. A natural product is a chemical that is produced by a plant, animal, or microorganism. Terrestrial plants and microorganisms have traditionally been an important source of natural products for the development of new drugs. Over 80% of the medicines most commonly prescribed in hospitals have their roots in natural products. These include compounds such as aspirin, which is a stable derivative of a compound present in the bark of the willow tree; numerous antibacterial drugs, such as penicillin, streptomycin, and erythromycin, which are produced by microorganisms; and the anticancer drug Taxol^®, derived from the bark of the Pacific yew tree.