Having taken a two-chapter detour into aspects of control and organization in electronic music, we return to describing audio synthesis and processing techniques. So far we have seen additive and wavetable-based methods. In this chapter we will introduce three so-called modulation techniques: amplitude modulation, frequency modulation, and waveshaping. The term ``modulation" refers loosely to any technique that systematically alters the shape of a waveform by bending its graph vertically or horizontally. Modulation is widely used for building synthetic sounds with various families of spectra, for which we must develop some terminology before getting into the techniques.