Different types of astrophysical jets are briefly described and the wide differences in their observed properties are emphasized. Various physical mechanisms that have been proposed for creating jets, based on gas dynamical, magnetic, and radiative stresses are briefly reviewed. It is argued that magnetic fields are probably generally important for collimating astrophysical jets.

Since nearly all discrete radio sources of astronomical interest are of insufficient angular extent for their detailed structural properties to be accessible to single-dish radio telescopes, radio interferometry must be employed to gain information on the morphologies of these objects. Recently constructed imaging interferometer arrays which employ the technique of Fourier synthesis, particularly MERLIN and the VLA (Very Large Array), and the more recent VLBI arrays, have given unprecedented imaging capabilities, with the result that our knowledge, and hence perceptions, of discrete radio sources have vastly changed over the last few years. An equally important parallel development has been image processing algorithms. These have vastly improved the quality of information produced by these arrays, so that an instrument such as the VLA can now produce images with speed and quality exceeding original design specifications by factors of 100 to 1000.

The subject of this paper is the interpretation of extragalactic radio jets. In this paper I will focus on what we have learned about the nature of extragalactic jets on the basis of model calculations. By model I mean any set of calculations, whether analytic, semi-analytic or numerical, which, when carried through from their respective assumptions to their internally self-consistent conclusions, help place constraints on the physical parameters and processes in the jets and their associated radio lobes. In this field, a visual inspection of a modern high-resolution radio interferometric observation (see review by PERLEY in these proceedings) often leads to statements like “that looks just like such and such in Landau and Lifschitz; I betcha that’s what’s going on!” This I call a speculation, or, at best, a hypothesis. I am addressing here the step beyond hypothesis, namely modeling, which is necessary to confront not only the object in question, but more importantly, the hypothesis itself. In the end, we will remember only the hypotheses.

This paper will be a short review of the topics covered at this conference on “Radiation Hydrodynamics in Stars and Compact Objects.” Rather than attempt to cover all the talks and posters, I will describe what I saw as the main themes and then summarize selected topics that provided unsolved questions. Finally, I will try to decide whether we strayed too far from the original purpose of the meeting. Or, put as a question: Does Radiation Hydrodynamics play a major role in solving a wide range of astrophysical problems?

As described in Mihalas’ introductory talk, radiation can affect an astrophysical plasma, flow, or object in several fashions. First, it can influence the kinematics through ionization, dissociation, and other gas-phase processes that depend on chemistry, ionization state, or excitation. But second, it can influence the dynamics of the flow, through deposition of momentum. Examples of the latter are shock waves, accretion disks, novae, supernovae, and hot-star winds. One can, therefore, divide the topics at this meeting into “active” and “passive” classes of radiation hydrodynamic problems (Table 1). The term “active” implies dynamic pressure from the radiation, whereas the term “passive” allows the radiation to affect the state of the gas, particularly its spectral emissivity, without affecting its motion appreciably.