In order to explain its variation with temperature and frequency the dielectric constant has been defined as a measure of the work done by an external electrical field in orienting molecules against the effect of their thermal agitation. Quantitative studies of the temperature variation of the dielectric constant of particular kinds of systems have been made and have given quite valuable information concerning the structure of molecules, but corresponding studies of the frequency variation of the dielectric constant have not as yet been so successful. It is the purpose of this article to discuss in some detail what seems to us to be the most important theoretical difficulty. By way of introduction it may be stated that in the case of a polar liquid there is a region in which the dielectric constant decreases with increasing frequency, the frequency region being dependent upon the size of the orienting molecule and the viscosity of the medium. The difficulty to be considered has to do with the manner in which in a solution the actual frictional resistance of the solvent molecules to the rotation of the polar solute molecules has been determined.