A wide range of important physical and chemical phenomena may be named by the same title: Rate processes in condensed media. They have the same underlying physics and mathematics. To these phenomena belong: 1) Small polaron motion [a] 2) Electron transfer between ions in solutions [b] and in photosynthetic centers [c]. 3) Electronic energy transfer between molecules or ions in solids and in liquids [d]. 4) Enzymatic catalysis [e] 5) Group transfer in biological systems [fl. 6) Electron-hole recombination in semiconductors [g]. 7) Non-radiative electronic relaxation in ionic centers Ih] and in impurity states in insulators [i]. 8) Recombination in amorphous solids [j]. 9) Radiationless transitions in large molecules [k]. At present a unified theoretical and conceptual framework exists for description and understanding of all these diverse phenomena. Our aim in writing this work is to introduce the student and the research worker in chemical physics to the main aspects of the conceptual framework of rate processes in condensed media. There exists an extensive literature devoted to various rate processes in condensed media. Among recent works, the books of Levine and Jortner [1], Fong [m] and Ulstrup [n] should be mentioned. The existence of this literature enables us to concentrate on the major theoretical aspects, omitting description and presentation of experimental data. We tried to make our work self-contained. Almost all the information necessary for reading the book is included. References used in the work by no means cover the literature on the subject.