The signatures of retardation in nonlinear optical susceptibilities are studied by starting with the multipolar Hamiltonian and using path integral techniques to develop a perturbative scheme for incorporating the retarded interaction with the electromagnetic field. The present approach accounts for the cooperative radiative decay and polariton transport which show up in resonant spectroscopies of nanostructures. These effects, which require a quantum electrodynamical description of the field, are missed by conventional nonretarded theories. Application is made to the enhanced spontaneous emission rate of biexcitons, which may show up in the nonlinear reflection off molecular superlattices.