1) Harmonics are voltages or currents that are integer multiples of the fundamental power line frequency, and they are caused by non-linear loads that distort the sine wave. 2) Harmonic currents flow at frequencies that are multiples of the fundamental frequency, adding in a distorted way to the current waveform. 3) Harmonics can interfere with equipment, require oversizing of electrical systems, and reduce the load carrying capacity of power systems.