A method which allows the dimensionally correct transformation of equations from one unit system to another is described. Three sets of transformations are explicitely given for the passage between (1) the Gaussian and the mks system, (2) the mks and a natural rationalized system with e=l, and (3) the mks and a natural rationalized system with c=l, h=l. © 1968, American Association of Physics Teachers. All rights reserved.