[en] The boson nature of radiation is shown to give rise to a purely quantum mechanical exchange contribution to the intensity-intensity correlation function for resonant light scattering by an atomic or molecular system. The exchange contribution can be decomposed into three components, one involving the intensity correlation for a pair of coherently scattered photons ('resonant Raman' processes), another for a pair of incoherently scattered ones ('resonance fluorescence'), and the last involving the exchange correlation one of each. The intensity correlation measurements of Kimble et al on optically pumped atomic beams of sodium atoms are interpreted with the theory, producing values of the decay rate of the excited sodium atoms and the coherence time of the exciting radiaton in good agreement with expectations