The paper deals with the magnitude and distribution of harmonic currents flowing in a.c. networks. The various sources of harmonics are discussed in general, and harmonics arising from the use of mercury-arc rectifiers and in h.v. transmission are dealt with in greater detail.The flow of harmonics arising from these sources is considered from a theoretical aspect, using equivalent networks, experimental verification of these theories being given for certain simple cases. The possibility of a complete theoretical solution is shown, and certain probable practical conclusions are drawn.Various methods of reducing harmonics are discussed, and the relative importance of the various sources is considered in the light of modern trends in harmonic suppression.