Phenolic acids, one group of commonly occurring polyphenolic compounds in foods, are classified into hydroxybenzoic and hydroxycinnamic acids which include gallic, and protocatechuic as well as caffeic and ferulic acids, respectively. They have demonstrated strong antioxidant and health beneficial properties especially for various degenerative diseases such as cancers and cardiovascular diseases in in vitro and in vivo models. However, phenolic acids are shown to undergo methylation, sulfation and glucuronidation during metabolism, with consequent changes of their structures and biological activities. In order to reveal a relationship between parent phenolic acids and their metabolites, intestinal, hepatic and microbial metabolisms as well as metabolites are reviewed in both in-vitro and in-vivo models. The importance of metabolites of phenolic acids and their biological activities are discussed.