The interstellar medium (ISM), which had formerly been better known for the allegoric shapes of its nebulae and dark clouds than for its physical properties, has developed into a fascinating area of astrophyscical research during the past few decades. An important aspect is the turbulent flows observed in many regions of the ISM, with velocities that, at least in the cooler parts, far exceed the speed of sound. These flows seem to play a decisive role in the cloud dynamics, slowing down gravitational contraction and star formation. In contrast to the objects studied in the two previous chapters, which had well-defined physical and geometrical properties and thus allowed a detailed analysis, the ISM is a rather diffuse system, whose modeling is far more uncertain and arbitrary. While the various atomic processes, such as transition probabilities and excitation rates, are well known and also the thermodynamic properties are fairly well understood, we have only a coarse picture of their hydrodynamics, including the effect of magnetic fields. Thus it is, for instance, difficult to apply hydrodynamic-stability theory without very special assumptions regarding geometry and flows. Hence the results we discuss in this chapter are mainly of qualitative nature, in which general arguments, such as equipartition and virialization, play an important role. The precise numerical factors, which are often found in the astrophysical literature, imply special choices of geometry and profiles and should not be taken too literally. In Section 12.1 we give a brief overview of the characteristic properties of the ISM.