  We derive a non-linear supersymmetric sigma-model for the transport of light
(classical waves) through a disordered medium. We compare this model with the
well-established non-linear sigma-model for the transport of electrons
(Schroedinger waves) and display similarities of and differences between both
cases. We show that for weak disorder, both models are equivalent (have the
same effective Lagrangean). This effective Lagrangean correctly reproduces the
(different) Ward identities for Schroedinger waves and for classical waves.
