In 1928 Chandrasekhara Raman reported a process in which a material would simultaneously absorb one photon and emit another. The energies of the two photons differed by an amount corresponding to the energy difference between two quantum-mechanical levels of the medium. Raman scattering, as the phenomenon came to be known, provided a tool for the spectroscopic investigation of energy levels not accessible by the usual absorption and emission techniques. For the first thirty-five years Raman scattering was a laborious and exotic technique, important more for the quantum-mechanical principles it illustrated than for its practical applications.