Electron transfer, one of the simplest of chemical events, profoundly affects chemical reactivity by inverting normal electron densities in an electron donoracceptor pair, thus activating previously inaccessible reaction modes. To the extent that most energy, and all life, on earth derive ultimately from photosynthesis, our very existence depends on this fundamental process. It is hardly surprising, then, that examples of electron-transfer activation abound within every subdiscipline of science. It is virtually impossible to peruse either general or specialized scientific journals without encountering at least one or two articles dealing with the topic. This very diversity attests to the importance of an informative overview of the subject but simultaneously underscores the difficulty of providing a satisfactorily comprehensive summary. © 1992, American Chemical Society. All rights reserved.