Phenolic acids are by their structure simple phenols and include two major subgroups: hydroxybenzoic acids and hydroxycinnamic acids. Phenolic acids are commonly present under two principal forms in all plant-derived foods: a free and a bound form. The latter is found more frequently and occurs in the form of esters, glycosides and bound complexes. In olives and olive oil one of the major component is simple phenolic hydroxytyrosol, (3,4-dihydroxy-phenylethylalcohol) which is a catechol derivative of oleuropein, the main component in olives.