Natural products are a source of chemical diversity and are an ideal starting point for most screening programs for pharmacologically active small molecules. Natural products can be extracted from tissues, cells, and secretions of microorganisms, animals, and plants. Crude extracts from any of these biological sources will contain a wide range of structurally diverse and novel compounds. Chemical diversity in nature is based on biological diversity. Pharmacognosy can offer the tools to identify, select, and process natural products destined for drug discovery. Usually, the natural product compound has some form of biological activity and that compound is known as the active principle - such a structure can evolve to become a discovery "lead".