This chapter contains sections titled: Introduction Radiation Theory Simple Harmonic Radiator Quantum Description Selection Rules Einstein Coefficients Harmonic Perturbation Blackbody Radiation Dipole–Dipole Energy Transfer Energy Levels in Atoms Crystal Field Splitting Acknowledgement References Introduction Radiation Theory Simple Harmonic Radiator Quantum Description Selection Rules Einstein Coefficients Harmonic Perturbation Blackbody Radiation Dipole–Dipole Energy Transfer Energy Levels in Atoms Crystal Field Splitting Acknowledgement References