  The Ehlers-Ellis 1+3 formulation of covariant hydrodynamics, when
supplemented with covariant radiative transport theory, gives an exact,
physically transparent description of the physics of the cosmic microwave
background radiation (CMB). Linearisation around a Friedmann-Robertson-Walker
(FRW) universe provides a very direct and seamless route through to the linear,
gauge-invariant perturbation equations for scalar, vector and tensor modes in
an almost-FRW model. In this contribution we review covariant radiative
transport theory and its application to the perturbative method for calculating
and understanding the anisotropy of the CMB. Particular emphasis is placed on
the inclusion of polarization in a fully covariant manner. With this inclusion,
the covariant perturbative approach offers a complete description of linearised
CMB physics in an almost-FRW universe.

Astrophysics