AbstractThe empirical parameters of the polarity of solvents which have been determined recently are reviewed: Y‐values (Winstein, Grunwald), X‐values (Gielen, Nasielski), Ω‐values (Berson, Hamlet, Mueller), Z‐values (Kosower), ET‐values (Dimroth, Reichardt, Siepmann, Bohlmann), R‐ and S‐values (Brownstein). Their field of application and interrelationships are discussed. The theoretical principles of the solvent effect on the reaction rate and light absorption of organic compounds are briefly described, and mention is made of attempts to correlate the physical characteristics of the solvent with the empiri