Good practice in basic research and industrial applications necessitates the accurate and regular calibration of circular dichroism instruments and synchrotron radiation circular dichroism beamlines for magnitude, optical rotation and wavelength. If spectra obtained in different laboratories are to be comparable and usable with standard reference datasets for secondary structure analyses, then instruments must be standardized, enabling cross-validation of spectra produced. In this chapter calibration methods are discussed along with techniques for measuring other relevant parameters such as sample cell pathlengths and protein concentrations. Accurate knowledge of the values of these parameters is important for producing correct spectral magnitudes, that are, in turn, essential for correct secondary structural analyses.