In Chap. 3 we considered the free oscillation of an electron on a spring whose oscillations decay because of radiative damping. We also treated forced operation. In this chapter we consider sustained oscillators. These systems do not decay (although they do dissipate energy), and they choose their own oscillation frequencies in distinct contrast with the forced oscillators of Sec. 3-2. Examples of these devices are clocks, triode oscillator circuits, and, to some degree of approximation, masers and lasers.