On account of its sensitivity to chirality, Raman optical activity (ROA), measured here as the intensity of a small, circularly polarized component in the scattered light using unpolarized incident light, is a powerful probe of protein structure and behavior. Protein ROA spectra provide information on secondary and tertiary structures of polypeptide backbones, backbone hydration, and side chain conformations, and on structural elements present in unfolded states. This article describes the ROA technique and presents ROA spectra, recorded with a commercial instrument of novel design, of a selection of proteins to demonstrate how ROA may be used to readily distinguish between the main classes of protein structure. A principal component analysis illustrates how the many structure-sensitive bands in protein ROA spectra are favorable for applying pattern recognition techniques to determine structural relationships between different proteins.