While the major constituents of essential oils are generally mono- and sesquiterpenes, in certain plant families or genera phenylpropanoid compounds are also found in the essential oil, sometimes as the main component. A survey is given of the distribution of these substances, as well as of their structural properties and biochemical origin. In particular, cinnamic acid not only occurs in some cases as a constituent of essential oils, but together with p-hydroxycinnamic acid is the most important metabolic intermediate in the pathways leading to the volatile phenylpropanoids found in these essential oils. These compounds are mostly phenolics, and often occur as their non-volatile glycosides. However, after enzymatic cleavage, which happens frequently during catabolic or postmortal processes, the resulting aglycones, which are not normally components of the essential oil, act as the carriers of flavors and fragrances. These compounds too are included in the survey.