Intravascular ultrasound (IVUS) provides direct depiction of coronary artery anatomy. Traditional use of this tomographic imaging modality has been in determination of geometric measurements of an artery, such as lumen or plaque size. However, by analyzing the backscattered or radiofrequency (RF) data it is possible to glean information on the composition of plaques. This chapter describes the theory of spectral analysis and its application clinical practice.