Circular dichroism (CD) is the differential absorption of the left- and right-circularly polarized components of plane-polarized electromagnetic radiation. It can provide structural and dynamic information about biological macromolecules, particularly proteins. The CD spectra in the far-UV (typically 180-240 nm) can give reliable quantitative estimates of the proportions of secondary structural features (helix, sheet, turn, etc.) present in proteins. The spectra in the near-UV (260-320 nm) can be used to explore the environments of aromatic amino acid side-chains and hence to give a measure of tertiary structure. Although CD cannot provide the high-resolution structural data available from X-ray crystallography or nuclear magnetic resonance, its convenience and applicability under a wide variety of experimental conditions make it the technique of choice in many applications, including exploring protein-ligand interactions, conformational changes and protein folding.