Cavity enhanced spectroscopy (CES) methodology provides a much higher degree of sensitivity than that available from conventional absorption spectrometers. The aim of this chapter is to present the fundamentals of the method, and the various modifications and extensions that have been developed. In order to set the stage, the limitations of traditional absorption spectrometers are first discussed, followed by a description of cavity ring-down spectroscopy (CRDS), the most popular CES embodiment. A few other well-known CES approaches are also described in detail. The chapter concludes with a discussion of recent work on extending CRDS to the study of liquids and solids.