  Modeling the atmospheres of SNe~Ia requires the solution of the NLTE
radiative transfer equation. We discuss the formulation of the radiative
transfer equation in the co-moving frame. For characteristic velocities larger
than about 2000 km/s, the effects of advection on the synthetic spectra are
non-negligible, and hence should be included in model calculations. We show
that the time-independent or quasi-static approximation is adequate for SNe~Ia
near maximum light, as well as for most other astrophysical problems; e.g., hot
stars, novae, and other types of supernovae. We examine the use of the Sobolev
approximation in modeling moving atmospheres and find that the number of
overlapping lines in the co-moving frame make the approximation suspect in
models that predict both lines and continua. We briefly discuss the form of the
Rosseland mean opacity in the co-moving frame, and present a formula that is
easy to implement in radiation hydrodynamics calculations.
