Circular dichroism (CD) spectroscopy is a powerful tool for the secondary structure analysis of proteins. The structural information obtained by CD does not have atomic-level resolution (unlike X-ray crystallography and NMR spectroscopy), but it has the great advantage of being applicable to both nonnative and native proteins under a wide range of solution conditions containing lipids and detergents. The development of synchrotron-radiation CD (SRCD) instruments has greatly expanded the utility of this method by extending the spectra to the vacuum-ultraviolet region below 190 nm and producing information that cannot be obtained using conventional CD instruments. Combining SRCD data with bioinformatics, molecular dynamics simulations, and other polarization techniques, such as linear dichroism and fluorescence anisotropy, provides new insights into the conformational changes of proteins in a membrane environment.