This chapter considers the complementary phenomena of overtones and partial signals that are present in technological low-pressure and atmospheric-pressure plasma jets. The term “technological” is used to define plasmas that are designed for the manufacturing of semiconductor devices [1], modification of engineering surfaces [2], and the treatment of biological material [3]. Equivalent electrical models that describe the generation of integer harmonics of the fundamental drive frequency (fo ) have been considered for many years and are now a means of plasma process control [4–9]. The physical origin of these plasma harmonic arises from the rapid oscillatory momentum of electrons and ions within the imposed electric field of the plasma ion sheath(s) that separates the plasma bulk from the reactor walls [10–13]. The plasma may also be considered to be a nonlinear power device and therefore standard radio frequency (RF) terminology is used throughout this chapter to describe the observations and measurements. Indeed, the harmonic distortion of a pure tone sinusoidal signal may be compared with the behavior of a thermionic valve or diode valve * and the solid-state diode mixer. More recently, a number of papers have proposed that low-frequency (kHz) generated harmonics may also be used to impart energy into the plasma [14,15].