Circular dichroism, CD, spectra can be used to gain information about quadruplex structures of DNAs as well as the effects of sequence, cations, chemical modification and ligand binding on quadruplex structure. There is not yet a validated approach to calculate a CD spectrum from a quadruplex structure nor is their one to go from a CD spectrum to a structure. However, it is possible to empirically correlate CD spectra features with quadruplex structural type in many cases. In this article four case studies are presented to indicate the strengths and limitations of CD in investigations of the properties of quadruplex structures formed by telomere repeat sequences. The case studies include determination of the quadruplex structural type present as a function of potassium concentration, the effect of sequence on the equilibrium between quadruplex structural types as a function of potassium concentration, the effect of ligand binding on quadruplex structure and the effect of 5' phosphorylation on quadruplex structural type.