  (shortened version)
  We show that many observations of W44, a supernova remnant in the galactic
plane at a distance of about 2500 pc, are remarkably consistent with the
simplest realistic model. The model remnant is evolving in a smooth ambient
medium of fairly high density, about 6 cm^-3 on average, with a substantial
density gradient. At the observed time it has an age of about 20,000 years,
consistent with the age of the associated pulsar, and a radius of 11 to 13 pc.
Over most of the outer surface, radiative cooling has become important in the
post shock gas; on the denser end there has been sufficient compression of the
cooled gas to develop a very thin dense half shell of about 450 M_sun,
supported against further compression by nonthermal pressure. The half shell
has an expansion velocity of about 150 km s^-1, and is bounded on the outer
surface by a radiative shock with that speed.
  We provide several analytic tools for the assembly of models of this type. We
review the early evolution and shell formation analyses and their
generalizations to evolution in a density gradient. We also calculate the
density and temperature that should be present in the hot interior of a remnant
with thermal conduction. We supply the van der Laan mechanism in a particularly
useful form for the calculation of radio continuum from radiative remnants.
Finally, we demonstrate a simple technique for estimating the optical emission
expected. These tools are employed to choose parameters of models which we then
explore with our 1d and 2d hydrocodes, providing, respectively, the detailed
x-ray spectra and dynamical characteristics.
