The general principle of the rationalization of electrical units was first stated by Heaviside in 1892, but for over forty years the matter has been under suspended judgment.In 1935 and 1938 the International Electrotechnical Commission considered the matter of rationalization but found itself so divided that it was unable to reach a decision about it. The paper seeks to restate Heaviside's arguments in modern terminology and language, in a form accessible to any professional electrical engineer, and to add certain new arguments in support of rationalization.The difference between rationalization of the M.K.S. system, as now proposed, and Heaviside's original method of rationalization, which involves a change in the size of the unit of current, is discussed.A number of examples of the effects of rationalization, and a table of rationalized units and constants, are provided. A list of references to Heaviside's published works concludes the paper.