During the past year, a number of developments have increased substantially our understanding of the coordinated relationship of human purine and pyrimidine metabolism and the role of specific aberrations of metabolism in this area in causation of human disease. However, in order to understand and appreciate the significance of these developments, background information is required. Since the subject has not previously been reviewed in this series, the considerable material relating clinical and biochemical correlations needs to be presented. A comprehensive description with specific citations of all major contributions of even the recent years is not possible within the space limitations of this chapter. Therefore, the information will be summarized largely by referencing more comprehensive reviews and more recent reports, rather than crediting, in all cases, the original citations of the various observations.