Although it is generally true that thermodynamics do not influence kinetics, this is NOT the case for electron transfer reactions in solution. Marcus Theory explains why this is so, using straightforward physical chemical principles such as transition state theory, Arrhenius' Law, and the Franck-Condon Principle. Here the background and applications of Marcus Theory are presented, with special emphasis given to biological redox reactions. Interesting conclusions that arise from the theory are also discussed. These include the nonlinear contribution of reorganization energy ([lambda]) to the activation free energy ([delta]G[superscript double dagger]), as well as the existence of a defined "normal range" of reaction cell potential ([delta]E[degrees]): Only if [delta]E[degrees] is within this range does a reaction of lower [lambda] have a lower [delta]G[superscript double dagger]; outside of this range, the lower-[lambda] reaction will have the higher [delta]G[superscript double dagger]. (Contains 1 table, 11 figures, and 2 schemes.)