  Resonance energy transfer has become an indispensable experimental tool for
single-molecule and single-cell biophysics. Its physical underpinnings,
however, are subtle: It involves a discrete jump of excitation from one
molecule to another, and so we regard it as a strongly quantum-mechanical
process. And yet, its kinetics differ from what many of us were taught about
two-state quantum systems; quantum superpositions of the states do not seem to
arise; and so on. Although J. R. Oppenheimer and T. F\"orster navigated these
subtleties successfully, it remains hard to find an elementary derivation in
modern language. The key step involves acknowledging quantum decoherence.
Appreciating that aspect can be helpful when we attempt to extend our
understanding to situations where F\"orster's original analysis is not
applicable.
