  The fast shocks that characterize supernova remnants heat circumstellar and
ejecta material to extremely high temperatures, resulting in significant X-ray
emission. The X-ray spectrum from an SNR carries a wealth of information about
the temperature and ionization state of the plasma, the density distribution of
the postshock material, and the composition of the ejecta. This, in turn,
places strong constraints on the properties of the progenitor star, the
explosive nucleosynthesis that produced the remnant, the properties of the
environment into which the SNR expands, and the effects of particle
acceleration on its dynamical evolution. Here I present results from X-ray
studies SNRs in various evolutionary states, and highlight key results inferred
from the thermal emission.
