In recent years there has been intense interest on the potential health effects of dietary polyphenols. Polyphenols are found ubiquitously in plants and are therefore abundant in human diet. Increased polyphenol consumption has been associated with a reduced risk of development of a range of chronic diseases, such as cancer, cardiovascular and neurodegenerative disorders. Initially the antioxidant property of polyphenols was believed to underlie their beneficial effects in vivo. However, they are subject to extensive metabolism in the small intestine, the liver and in the colon following oral ingestion and the resulting circulating metabolites have reduced antioxidant potential. Despite this, other potential mechanisms of action have emerged for polyphenols, which include their interaction with cell signalling pathways and modulation of mitochondrial fuction. In this Chapter we aim to: 1) provide an overview of the different classes of polyphenols, 2) to describe their biosynthesis within plants, 3) to provide an understanding of the metabolism and biotransformation of polyphenols within the body following ingestion and 4) to highlight their potential mechanisms of action in the body, notably their antioxidant and non-antioxidant activities.