In this paper we explore the foundations and some applications of the energy gap law (EGL) for nonradiative and radiative charge recombination from an ion pair state to the ground electronic state of isolated (solvent-free) and solvated donor (D)-acceptor (A) complexes and DBA bridged (B) supermolecules. The energy gap dependence of the averaged Franck-Condon density AFD(E), which is proportional to the microscopic electron-transfer (ET) rate, k(E), at the excess energy E, was calculated numerically (for a range of E) and by saddle point integration (for E = 0) for a displaced harmonic potential system. The intramolecular electron vibration coupling parameters were inferred from resonance Raman data and from ET emission line shapes. For isolated supermolecules an energy gap (Delta E) dependence of AFD(E) was derived, which for the electronic origin (E 0) is a multi-Poissonian, with a Gaussian dependence over a narrow, low Delta E domain and a superexponential decrease with increasing Delta E for large Delta E. The EGL, AFD(0) = A exp(-gamma Delta E), holds for large values of Delta E over physically relevant Delta E domains (of similar to 5000 cm(-1)), where the theoretical parameters gamma and A have to be extracted from numerical calculations using a complete set of nuclear frequencies and their coupling parameters.