The word aromatherapy (aroma means fragrance and therapy means healing) has been used with various meanings and definitions in our day to day life. But the most common and widely accepted definition is that used by The Collins English Dictionary Millennium Edition, 2000. According to this, aromatherapy is “The use of fragrant essential oils extracted from plants as a treatment in alternative medicine to relieve tension and cure certain minor ailments.” Alternative medicine uses a variety of therapeutic or preventive health care practices like homeopathy, chiropractic, and herbal medicine that do not follow generally accepted medical methods and may not have a scientific explanation for their effectiveness. According to Clare Walters Aromatherapy is the use of therapeutic oils extracted from natural plant matter in order to encourage good health, equilibrium and well-being. Chrissie Wildwood defines Aromatherapy as a a multifaceted art which uses essential oils of aromatic plants and trees to promote health of body and calmness of mind. Some people call it a sub-class of a broader area of medicine called HERBAL MEDICINE. THIS PEER-REVIEWED ARTICLE IS PUBLISHED IN BRAINMASS.COM. The word aromatherapy was discovered in 1928 by a French Chemist René Maurice Gattefossé when he met with an accident while working in his laboratory. He found the therapeutic use of aromatic substances (essential oils) following the use of lavender (a plant belonging to mint family) essential oil to cool a severe burn he suffered in that accident. In 1937 the first book on aromatherapy Gattefossé ‘s Aromatherapy appeared in French.The modern day expert in aromatherapy Robert Tisserand is the first author of aromatherapy in English “The Art of Aromatherapy). Since aromatherapy involves essential oils a curious question is WHAT ARE ESSENTIAL OILS. The word essential oil is derived from QUINTESSENCE which means an extract of a substance containing its principle constituent in its most pure and concentrated form. Essential oils are highly volatile and evaporate quickly if exposed to open air. When left in open air the oils create an aromatic scent which brings the effects associated with aromatherapy. The structure and chemical composition of an essential oil is dependent on the the plant variety and the geographical location of the plant. Several varieties of eucalyptus oil extracted from rosemary,mint leaves are used in aromatherapy. The properties of oils like fragrance, color, degree of volatility and the mode of action depends on the chemical structure. of the ways in which that oil interacts with the human body. The most common constituents of essential oils are terpenes, alcohols, aldehydes and esters. It is these chemical combinations that determine the fragrance, the color, the degree of volatility, and the therapeutic properties of the essential oil. Terpenes are the low molecular weight isoprene compunds that are responsible for the fragrance of essential oils. Due to the presence of combination of chemical compounds in essential oils it is not surprising to expect variety of functions exhibited by them e.g analgesic,anti inflammatory,psychological and physiological. They enter the human body much easily via skin or by inhalation due to small molecular weight and hydrophobic(water-hating). The mode of action of essential oils has been studied mainly in two ways. According to first mechanism the olfactory system( nose) smells the oil and sends signal to the limbic system (brain) thus bringing out the psychological healing effect. The second one is the pharmacological method in which the oil is applied and used as a medicine for treatment.