Mathematical treatments of periodic functions and a.c. circuit are presented and then some typical periodic concentration polarizations are reviewed as diffusion problems, the solutions of which are derived by means of the Laplace transformation. The topics discussed here are : The experiments shown by Fournier, Kalousek and Ishibashi with Fujinaga, a.c. and square wave polarography, oscillographic polarography of Heyrovsky-Forejt type and a.c. chronopotentiometry.