Harmonics cause distortions of voltage and current waveforms, which have adverse effects on electrical equipment. Harmonics are one of the major power quality concerns. This chapter discusses the nature of harmonics and their generation by electrical equipment. The most common harmonics in power systems are sinusoidal components of a periodic waveform that have frequencies that can be resolved into some multiples of the fundamental frequency. The arc furnace loads are highly polluting and cause phase unbalance, flicker, impact loading, harmonics, and resonance, and may give rise to torsional vibrations in rotating machines. Concerns for harmonics originate from meeting a certain power quality, which leads to the related issues of effects on the operation of electrical equipment, harmonic analysis, and harmonic control. These harmonics can be considered to produce, by an additional set of rotating poles, rotor EMFs, currents, and harmonic torques akin to the fundamental frequency; but synchronous speeds depending upon the order of the harmonics.