A general theory of spectroscopic interferences for predissociating systems is developed using scattering theory. The introduction of complex bound states with complex eigenvalues results in a simple method for the derivation of total absorption, predissociation, and photon scattering (Raman) cross sections. The validity of other formalisms for these interferences, including Fano lineshapes, is examined and cases of spectroscopic interest are pointed out.