Essential oils (EOs) are extremely complex mixtures containing compounds of several different functional-group classes. A specific aromatic profile should be determined by gas-chromatography-mass detection methods, to define standards for their safety and efficacy. The chemical constituents of the essential oils, their flavour and their taste act both alone and in synergy, always determining a global psychosomatic action. The main therapeutic activities of the EOs are reported as spasmolythic, revulsive, anti-inflammatory and decongestant, immunomodulant, antimicrobial, antimycotic, expectorant, mucolythic, antioxidant, psychotrope, analgesic and acaricide. The use, posology, route of administration as well as toxicity and adverse effects are reviewed.