Having taken a two-chapter detour into aspects of control and organization in electronic music, we now return to describing audio synthesis and processing techniques. So far we have considered additive and wavetable-based methods. In this chapter we will introduce three types of modulation, called amplitude modulation, frequency modulation, and waveshaping, all of which are useful for building synthetic sounds with certain, characteristic families of spectra. We will first need some terminology for speaking about spectra, which we introduce in the next section.