  We present near-infrared (NIR) spectra for Type Ia supernovae at epochs of 13
to 338 days after maximum blue light. Some contemporary optical spectra are
also shown. All the NIR spectra exhibit considerable structure throughout the
J-, H- and K-bands. In particular they exhibit a flux `deficit' in the J-band
which persists as late as 175 days. This is responsible for the well-known red
J-H colour. To identify the emission features and test the $^{56}$Ni hypothesis
for the explosion and subsequent light curve, we compare the NIR and optical
nebular-phase data with a simple non-LTE nebular spectral model. We find that
many of the spectral features are due to iron-group elements and that the
J-band deficit is due to a lack of emission lines from species which dominate
the rest of the IR/optical spectrum. Nevertheless, some emission is unaccounted
for, possibly due to inaccuracies in the cobalt atomic data. For some
supernovae, blueshifts of 1000--3000 km/s are seen in infrared and optical
features at 3 months. We suggest this is due to clumping in the ejecta. The
evolution of the cobalt/iron mass ratio indicates that $^{56}$Co-decay
dominates the abundances of these elements. The absolute masses of iron-group
elements which we derive support the basic thermonuclear explosion scenario for
Type Ia supernovae. A core-collapse origin is less consistent with our data.
