  We review the origin and structure of hot (cooling flow) gas in elliptical
galaxies. X-ray observations can be used to determine the stellar mass to light
ratio, the mass profiles of dark matter halos, and the interstellar magnetic
field. Interstellar gas cools over a large volume, forming stars with a
bottom-heavy IMF. For consistency with the thin fundamental plane, young stars
must be optically luminous. Circular X-ray isophotes in rotating elliptical
galaxies indicate distributed radiative cooling or strong interstellar
turbulence.
