(GC) are known to exhibit different physical properties and chemistry
than those in the outer Galaxy. The main puzzle has been to understand
the heating mechanisms which apparently affect only the gas, but not
the dust.  With the advent of ISO, our picture of the GC is changing
dramatically. Our SWS and LWS data show a completely new picture. We
now see, for the first time, large amounts of hot H2 and ionized gas
which require UV radiation with an effective temperature of 37000
K. These new components are however behind more than 50 mag of visual
extinction produced by the cold gas traced by the CO and the continuum
measurements at 60 and 145 microns (Odenwald and Fazio 1984, ApJ 283,
601).  Though these new results suggest that the heating might be due
to OB stars, the lack of recombination lines and compact continuum
sources in the GC does not fit within this picture. Other
possibilities would be large hot bubbles produced by massive evolved
stars or large scale shocks. At this stage, we really need to know the
spatial distribution of the ionized and hot neutral gas recently
revealed by ISO in the spectral line mode. These images will show the
relative distribution of the ionized and the hot neutral gas providing
us with the information which will allow to establish the nature of
the heating sources of the molecular clouds in the GC. Our
understanding of the properties of molecular clouds in the center of
the Galaxy is extremely important, with this information for a nearby
prototype, we have a unique comparison with more distant galaxies for
which our angular resolution and sensitivity are inadequate.