Unani System of Medicine is an important segment of AYUSH (Ayurveda, Yoga and Naturopathy, Unani, Siddha and Homoeopathy). In 20th and 21st century due to side effects of synthetic drugs, there is an increasing interest in ASU (Ayurveda, Siddha and Unani) medicine. At present the adulteration of the herbal drugs is the burning problem in ASU herbal industry and it has caused a major problem in the research on commercial natural products. The deforestation and extinction of many species and incorrect identification of many plants has resulted in adulteration and substitution of raw drugs. The future development and analysis of herbs is largely dependent upon reliable methodologies for correct identification, standardization and quality assurance of ASU drugs. In India normally the contamination/adulteration in food/crude drugs is done either for financial gain or due to carelessness and lack of proper hygienic conditions of processing, storing, transportation and marketing. Medicinal plants constitute an effective source of traditional and modern medicine. Adulteration is considered as an intentional addition of foreign substances to increase the weight of the product or to decrease its cost. It may be due to various factors like confusion in vernacular names, lack of knowledge about authentic plants, non availability of genuine drugs, similarity in morphology, activity, aroma, careless collection and other unknown reasons. This article throws a light on adulteration, types, common market adulterants in ASU medicines and prescribed Prevention methods.