Many plants synthesize substances that are useful to the maintenance of health in humans and other animals. These include aromatic substances, most of which are phenols or their oxygensubstituted derivatives such as tannins. Many are secondary metabolites, of which at least 12000 have been isolated a number estimated to be less than 10% of the total. In many cases, these substances (particularly the alkaloids) serve as plant defense mechanisms against predation by microorganisms, insects, and herbivores. Many of the herbs and spices used by humans to season food yield useful medicinal compounds. The demand for medicinal plants is currently increasing in both developed and developing countries for various reasons. For some it would be the growing recognition that natural products have fewer or even no side effects; for others it would be their accessibility and affordable costs that would tip the scales. Medicinal and aromatic plants are used as plants themselves or parts of plants or can be processed by distillation to become essential oils. They are used in pharmacy, cosmetology , perfumes and the food industry among others.