On the subject of the interstellar medium, there are vastly more questions than answers. There are few topics in astrophysics which are so well served by the new and planned generation of space-borne and ground-based observatories. These highly technological and expensive facilities provide the necessary impetus to encourage progress from theorists and computational analysts alike.
Although the principal components of the interstellar medium have been identified and many of their properties measured, there is very little understanding of how they fit together into a dynamic system. This lack of progress has begun to stifle progress in other astrophysical fields. What are the feedback mechanisms that determine the stellar and interstellar properties of a galaxy? What regulates the star formation rate? Why do the many components of the ISM have very roughly the same energy density? What is the volume filling fraction and topology of the various gas phases? What is the large-scale and fine-scale topology of the magnetic field for each of the gas phases? How do the properties of the ISM change with cosmic time as more and more of the gas becomes locked up in stars?
To quote from Spitzer (1990), ``Understanding the processes that occur as the hot interstellar gas evolves in our Galaxy is an ambitious goal that we are far from achieving.'' The dynamics of a compressible gas bombarded by photons and cosmic rays is a highly complex problem. Progress has been made through idealized models. In these models, one commonly recognizes three phases. Initially, the supernova ejects a rapidly expanding envelope whose interaction heats the ambient gas to high temperatures. Second, the hot gas expands and compresses or destroys clouds in its wake. Finally, the hot gas may escape or fall back down to the galactic plane. The problem is that, in practice, these three stages overlap so much that their mutual interactions are crucial. Since we can expect major gains in the computational power of supercomputers, we can anticipate the development of more realistic models in the decades ahead.