EOs of plant origin are an important product of agriculture-based industries. EOs and their components, particularly for their broad-spectrum antimicrobial activity, which can provide, for instance, alternative functional ingredients to extend the shelf life of food products and ensure microbial safety for consumersEOs may constitute 20–100 different plant secondary metabolites belonging to a variety of chemical classesAround 3000 EOs have been produced from at least 2000 plant species, out of which 300 are important from a commercial point of viewThey are usually stored in the oil ducts, resin ducts, glands, or trichomes (glandular hairs) of plants. the aim of this chapte is a review on the chemistry of essential oils and factors influceing their constituents