The aim of this chapter is to introduce the techniques of circular dichroism (CD) and synchrotron radiation circular dichroism (SRCD) spectroscopies. A brief account is given of how the spectrum of a molecule is generated as a consequence of excitations resulting in electronic transitions, which for chiral molecules produce differential absorbances for left- and right- handed circularly polarised light. There follows an overview of the important basic principles of CD and of good practice protocols for collecting data, areas which are further developed in subsequent chapters of this book. In addition there are sections describing potential applications of CD for studies of proteins and nucleic acids. The final section illustrates the enhanced capacity inherent in the technique of SRCD spectroscopy for applications in structural biology, again discussed in detail in other chapters.