The aim of this book is to give a simple and at some points detailed presentation of main ideas, concepts and theoritical approaches to charge transfer reactions kinetics in polar solvents. The presentation is given in a simple manner making reading easy even for those who want to enter into this field. A detailed discussion of the physical mechanism of elementary electron transfer reactions (used as a simple example) is given. The electron transfer between molecular species in the bulk of the solution and at the electrodes is discussed. The role of the dielectric properties of the solvent is considered. The difference between various tunnelling phenomena and their relevance to the electron transfer processes are shown. A brief discussion of stochastic approach to chemical kinetics is given as well as of an extention of theory to more complicated chemical reactions.