  We review recent progress in understanding the evolution of hot interstellar
gas in the halos of elliptical galaxies. Non-homologous variations in the
physical size of the hot gas account for the large variations of x-ray
luminosity among ellipticals of similar L_B. By combining ROSAT and Einstein
data we derive the distribution of total mass in NGC 4472. From 0.1r_e to 1r_e
the total mass is identical to the expected stellar mass. Therefore stellar
mass to light ratios can be determined from x-ray observations! Also the widely
used ``mass dropout'' assumption must be incorrect in this important part of
the cooling flow. Recent ROSAT observations indicate gas temperatures in excess
of the virial stellar temperature, totally unlike standard cooling flow models.
However, these new results can be understood if an additional massive component
of ``circumgalactic'' gas is assumed to fill the outer galactic halos beyond
most of the stars. This old hot gas, first heated during the epoch of galaxy
formation, continues to flow into the stellar parts of ellipticals today,
combining with gas expelled from evolving stars. This dual origin of hot
interstellar gas further complicates recent discussions of abundances in the
hot interstellar gas.
