Essential oil are very important and widely used since early times. The important role and mode of action of these naturally occurring products is discussed with regards to its bioactivity as antibacterial, antiviral, antioxidant and diabetic. Also its important role in chemoprevention and cancer suppression is discussed. The therapeutic properties of essential oils in aroma will be outlined. All these functions and its properties are due to the presence of certain constituents which are discussed in this review along with their chemical structures.