The theme of this Colloquium is radiation hydrodynamics in and around stars and other compact bodies. To open our discussions, I would like to offer some rather elementary remarks about the role played by radiation in astrophysics.

It is probably true that most astronomers view radiation primarily as a diagnostic tool. After all, the only access we have to astrophysical bodies (with a few exceptions inside the Solar System) is the photons we capture from them. And so an immense effort has been devoted to the development of techniques for converting raw information about the spatial, temporal, spectral, and polarization variation of the observed radiation field into knowledge about the physical structure of the object that produced the radiation. There are many difficult challenges, both observational and theoretical, to be met in this process, and the field is in a state of rapid development today, and will remain so for the forseeable future. Nevertheless, in the context of this conference it is worth emphasizing that in the diagnostic problem radiation plays an essentially passive role; it is merely the tool used to analyze the situation.