The electronic structure of the peptide bond is of central importance in understanding the optical properties of proteins. Recent developments in resonance Raman spectroscopy permit a new and more quantitative picture of this electronic excitation to be constructed. The information obtained will be of direct relevance to the theory of protein circular dichroism and the interpretation of resonance Raman spectra in terms of structural variations. The effect of solvation on the location of the npi electronic excitation of the peptide bond will be determined. Exciton models for the interaction of peptide groups will be directly tested for simple dipeptides. Generally applicable methods for the analysis of electronic excitations of complex chromophores will be developed and applied to the peptide bond and other chromophores present in proteins. A more complete description of the potential energy surface of the ground electronic state of the peptide linkage will also be derived in these studies. In particular, the effect of solvation on the force constants of the peptide groups will be determined. This is of importance to the construction of reliable potential energy surfaces used for protein structure and dynamics calculations. The need for reliable optical methods of structural analysis is greatest for those cases where dynamic interconversion between several conformational states is suspected. This is a common situation for oligopeptides of pharmacological interest and may be an important feature of loop structures of some proteins. Optical methods such as absorption, circular dichroism and resonance Raman spectroscopy provide an instantaneous picture of the ensemble of conformational states. The relative contribution of each of several conformers could be determined if their individual spectra could be calculated. NMR and x-ray diffraction methods provide average pictures. Methods of calculating the relative free energies of several conformational states could be tested for reliability if optical spectra could be calculated for each predicted conformer. Spectral methods, particularly CD spectra, are apparently sensitive to very small conformational changes but there is lack of ability to interpret the spectral changes in structural terms.