Tea and coffee are rich in polyphenols with a variety of biological activities. Polyphenols found in tea are predominantly flavonoids, of which up to 15% are present as free or esterified gallic acid. Coffee polyphenols are almost wholly comprised of chlorogenic acids. Many of the demonstrated activities of polyphenols are consistent with favourable effects on the risk of chronic diseases. In investigating the relationships between intake and exposure to such compounds and chronic disease-related endpoints, it is important to be able to identify biomarkers that are specific to the compounds of interest. 4-O-methyl gallic acid (4OMGA) and isoferulic acid have been identified as potential biomarkers of intake and exposure to polyphenols derived from tea and coffee, respectively. 4OMGA is derived from gallic acid in tea, and isoferulic acid from chlorogenic acid in coffee. The major objectives of the research which is the subject of this thesis were (1) to establish a dose-response relationship of 24h urinary excretions of 4OMGA and isoferulic acid following ingestions of black tea and coffee of different strengths, and (2) to explore relationships of tea and coffee intake with 24h urinary excretion of 4OMGA and isoferulic acid in human populations. It was found that there was rapid excretion of both 4OMGA and isoferulic acid in the first 6h after tea and coffee ingestion, respectively. Approximately 60 80% of the ingested dose was excreted during the first 6h after ingestion. Urinary excretion of 4OMGA and isoferulic acid was directly related to the dose of tea and coffee, respectively. That is, higher intake resulted in increased urinary excretion of the metabolites. The relationships of 24h urinary excretion of 4OMGA and isoferulic acid with long-term usual (111 participants) and contemporary recorded current (344 participants) tea and coffee intake were assessed. 4OMGA was strongly related to usual (r = 0.50, P < 0.001) and current (r = 0.57, P < 0.001) tea intake. Isoferulic acid was less strongly, but significantly associated with usual (r = 0.26, P = 0.008) and current (r = 0.18, P < 0.001) coffee intake. Overall, the results are consistent with the proposal that 4OMGA is a good biomarker for black tea derived polyphenol intake and exposure, but isoferulic acid may have only limited use as a biomarker for coffee-derived polyphenol exposure.