The ISM plays a vital role in the evolution and development of the Galaxy. It is the stuff from which new stars are born and it is the graveyard where dead stars are buried.
Some regions are very hot, very tenuous and highly ionised while others are cool, dense and neutral.
The quality and coverage of the present data give us the best view yet of the ISM within 200 parsecs of the Solar System. When this is complete the study of emission from more distant sources will allow us to extend the look out to 1000 parsecs or more. It is expected that forthcoming spectroscopic observations from the XMM Observatory will provide excellent detailed diagnostics of both the emission and absorption from the ISM and will tell us about the composition and ionisation state of material. They inject new stellar material and energy into the medium and the development of emission from supernova remnants acts as a probe of the interstellar environment surrounding the supernova. Detailed comparison of the X-ray emission with other wavebands from radio to optical gives us a powerful diagnostic of the physics of the shock.