In the last twenty years many peptides and proteins were introduced to the market for pharmaceutical use. The high complexity of the biomacromolecules requires the use not only of physicochemical methodologies but also of immunochemical and biological techniques for the characterization and quality control of these substances. Characterization of the active conformation and post-translational modifications is important for the understanding of the in vivo activity of peptides and proteins. Among other spectroscopic techniques circular dichroism has become a helpful tool for evaluating the secondary and tertiary structure of proteins and peptides. A combination of several analytical methods is necessary for a comprehensive characterization of the substance under investigation and for an appropriate quality control of the product concerning identity, purity and potency. Suitable analytical methodologies are discussed using erythropoietin as an example.