Resonances are ubiquitous in molecular heterojunctions and in scanning tunneling microscopy (STM) experiments. In the former environment, resonance tunneling is essential for favorable wire-length-dependence of the conductance and is often the mechanism underlying conductance enhancement through application of a gate voltage. In the latter environment, resonance tunneling has served to develop a powerful vibrational spectroscopy. Resonance conductance is often strongly nonadiabatic; in the course of the tunneling event, electron energy is channelled into vibrational modes and triggers molecular dynamics. The qualitative physics underlying current-driven, resonance-mediated dynamics in molecular electronics is very simple, and is familiar from related phenomena such as gas phase electron-molecule scattering and photochemistry on conducting surfaces. Equilibrium displacement between the initial and resonant states translates into vibronic coupling in the language of the Marcus theory of electron transfer; it produces a nonstationary superposition in the nuclear subspace that evolves during the resonance lifetime. Upon relaxation the system is internally excited and interesting dynamics is likely to ensue. While the underlying physics is very general, the single-molecule STM and molecular heterojunction environments open unique and exciting opportunities. The former introduces the possibility of determining resonance lifetimes through fit of experimental voltage dependencies to a quantum mechanical theory. The latter introduces the possibility of developing coherently driven molecular machines, a new form of nanolithography, and a new means of manipulating the conductivity of molecular-scale devices. We briefly review the theory of current-driven dynamics in molecular-scale devices, discuss the results of ongoing research on surface nanochemistry and molecular machines, and sketch a variety of potential applications.
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