Mixed valency is important in many areas of chemistry, from synthetic to biological systems, from the simplest systems to the most highly complex. The purpose of our research is to understand the principles of mixed valency in localized, nearly delocalized, and delocalized systems, and to apply our knowledge to the rational design Of Molecular devices. This microreview discusses our group's research over the last ten years. It reviews methods of estimating picosecond electron-transfer lifetimes from infrared spectral lineshapes, studies of the locatized-to-delocalized transition and the effects of frozen solvents, charge gating by non-covalent interactions, electron-transfer-gated electron transfer, and the distribution of charge within a molecule probed and controlled by all applied electric field. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weirtheim, Germany, 2009)