Hydrogen replacement reactions play an outstanding part in organic chemistry. They include such industrially important processes as halogenation, nitration, sulfona­ tion, alkylation, and direct amination and metallation of organic substances. It is no accident that the data obtained in the investigation of these reactions have been used extensivel y in the solution of a series of fundamental problems in theoretical organ­ ic chemistry and this tendency still exists. Isotopic exchanges of hydrogen also belong to this group of chemical conversions. They have attracted considerable attention as they are the simplest and, to some ex­ tent, models of hydrogen replacement reactions, which are convenient for elucidating the mechanism of hydrogen replacement and the effects of interaction between atoms in organic molecules. The mechanisms of hydrogen replacement reactions are examined in this monograph and the most space is allocated to hydrogen isotope exchanges. The mechanisms of all the given reactions are interpreted from the point of view of the acid­ base properties of the reagents. In this connection, a large section of the book is de­ voted to a thorough substantiation of the thesis that hydrocarbons participating in the given reactions act as acids or bases. Problems of acid-base catalysis and the mech­ anism of acid -base interaction are also discussed. Thus, a considerable part of the book is concerned with problems of the theory of acids and bases.