  We present a method, based on Feynman path integrals, to describe the
propagation and properties of the quantised electromagnetic field in an
arbitrary, nonlinear medium. We provide a general theory, valid for any order
of optical nonlinearity, and we then specialise the the case of second order
nonlinear processes. In particular, we show, that second-order nonlinear
processes in arbitrary media, under the undepleted pump approximation, can be
described by an effective free electromagnetic field, propagating in a vacuum,
dressed by the medium itself. Moreover, we show,that the probability of such
processes to occur is related to the biphoton propagator, which contains
informations about the structure of the medium, its nonlinear properties, and
the structure of the pump beam.
