Impedance and intensity are common terms used both in sound radiation from vibrating structures and in sound propagation. In this chapter, the various types of impedance are discussed in detail, followed by a discussion of both sound intensity for sound propagation in a gas (usually air) and structural intensity for sound waves propagating in structures. A number of similarities and differences may be observed for the two different types of intensity. Fundamentally, sound and structural intensity are both a measure of the energy that is propagating in the respective medium.