It is the study of the physical, chemical, biochemical and biological properties of drugs, drug substances of natural origin as well as the search for new drugs from natural sources. It deals especially with medicinal substances obtained from plants. Pharmacognosy is the study of drugs derived from natural sources. Pharmacognosy is the discipline of pharmacology with a special focus on the composition, use, and development of medicinal substances of biological origin and especially medicinal substances obtained from plants. It comprises of three subjects, botany, chemistry and pharmacology of the drugs from plants or simply the herbs. Botany includes taxonomy, genetics and the cultivation processes of the plants, chemistry includes isolation, identification and the quantification of the chemical constituents of plants and the pharmacology includes the study of the effects of the herbal drugs on biological systems. Phytochemical screening and bioassays in which the plant-derived substances have recently become of great interest owing to their versatile applications. Medicinal plants are the richest bio-resource of drugs of traditional systems of medicine, modern medicines, Nutraceuticals, food supplements, folk medicines, Pharmaceutical intermediates and chemical entities for synthetic drugs.