Harmonics have been present in power systems since the very early stages of AC system development. The principal reason for this is that when the battle between AC and DC systems was resolved in favour of AC systems the one piece of equipment which fought a rearguard battle was the DC motor. It could be controlled in a manner which AC motors could not, both torque and speed control were accurately attainable. The interface between the three phase AC system and the DC motor took the form of various types of converter. All forms however shared one feature, they all used some form of switch and thus they drew currents from the AC supply which were not sinusoidal. The author looks at the commonest source of harmonics and harmonic problems in power systems-the three phase Graetz bridge. This is widely used in motor drives of all types as well as HVDC transmission and such equipment as uninterruptible power supplies (UPS) so is discussed in some detail. The principles established also apply however to a wide range of other harmonic sources. (3 pages)