  Supernovae, with their diversity of compositions, velocities, envelope
masses, and interactions are good testing grounds for probing the importance of
NLTE in expanding atmospheres. In addition to treating H, He, Li I, O I, Ne I,
Na I, and Mg II in NLTE, we use a very large model atom of Fe II to test the
importance of NLTE processes in both SNe Ia and II. Since the total number of
potential line transitions that one has to include is enormous ($\approx 40$
million), approximations and simplifications are required to treat the problem
accurately and in finite computer time. With our large Fe II model atom (617
levels, 13,675 primary NLTE line transitions) we are able to test several
assumptions for treating the background opacity that are needed to obtain
correct UV line blanketing which determines the shape of near-maximum light
supernova spectra. We find that, due to interactions within the multiplets,
treating the background lines as pure scattering (thermalization parameter
$\epsilon = 0$) is a poor approximation and that an overall mean value of
$\epsilon \sim 0.05 - 0.10 $ is a far better approximation. This is true even
in SNe Ia, where the continuum absorption optical depth at 5000 \AA\ ($\equiv
\tau_{\rm std}$) is $<< 1$. We also demonstrate that a detailed treatment of
NLTE effects is required to properly determine the ionization states of both
abundant and trace elements.
