Individually, both HPLC and RIA are useful techniques for either the qualitative (HPLC) or the quantitative (RIA) analysis of peptides. In combination, however, the two techniques represent an extremely powerful tool for the precise and sensitive characterization of biologically important peptides in tissue or body fluid extracts. In this chapter our aim has been to provide the reader with some general guidelines on developing appropriate strategies for peptide characterization. We have also included practical examples of what can be achieved using these techniques. Absolute conditions must, of course, be determined empirically, depending on the biochemical nature of the peptide(s) and tissue matrix. However, it is hoped that this chapter may help in establishing a starting point for subsequent method development.