After both an introduction into the basic problems of electron transfer reactions (“redox reactions”) and their interconnection with substitution reactions, and a description of the situation met by Henry Taube in the early fifties, are presented and discussed: the so-called labile and inert complexes considering the kinetics of ligand exchange; the outerandinner- spheremechanisnisandtheir impact on electron transfer as well as the restrictions caused by the Franck-Condon principle (through potential-energy diagrams); the Marcus model for the outer-sphere mechanism; the important historical studies on the activated bridged Taube complex and their mechanistic consequences (precursor, activated, successor complex); and bridged mixed-valence complexes with corresponding model conceptions elaborated by Hush for the intervalence transfer bands in the electron spectrum of these complexes.