The functioning of almost all biological macromolecules depends on their ability to form specific interactions with one or more other species, such as other macromolecules or small molecule ligands, and quantitative characterization of these interactions often leads to a greatly improved understanding of the role of the particular protein or nucleic acid in its cellular context. All common biological macromolecules exhibit the phenomenon of circular dichroism (CD), the differential absorption of left-and right-handed circularly polarized light, and this CD signal can be exquisitely sensitive to the conformational state of the molecule. The fact that complex formation is frequently accompanied by conformational changes means that CD spectroscopy is one of the few techniques that allows one to address simultaneously both the structural and the energetic aspects of these interactions. Because each experimental system is different, and will inevitably present its own particular set of challenges, the process of determining an equilibrium dissociation constant cannot be presented as a single simple protocol that can be applied in every situation. In this chapter we therefore provide a general overview of CD-based approaches with emphasis on experimental design, data collection, and data analysis. We describe the methods that may be used when a suitable signal change accompanies formation of the complex as well as those that may sometimes be used when there is no signal change.