Spiral galaxies consist of three spatially and dynamically dinstinct components, namely bulge, disk, and halo. Of these, the central bulge consists primarily of old stars and has a low gas fraction. The disk and halo consist of stars plus gas, dust, and in many cases ``dark matter''. Here I want to concentrate on the interstellar medium (ISM) in the disks and halos of galaxies, with emphasis on late-type spirals. Massive dark halos will not be described (see e.g. Ashman 1992; Holt and Bennett 1995). In the current context ``late-type'' stands for disk-dominated spirals with rather inconspicuous bulges (Hubble types Sb - Sdm). The ISM of elliptical galaxies and early-type spirals (S0 - Sab) will also not be presented here (cf. Walsh et al. 1989 and Fabbiano 1989).
Studies of the ISM contribute significantly to our understanding of the structure, kinematics, and evolution of galaxies. The level of star formation (SF), its spatial distribution, and its temporal changes depend strongly on the properties of the interstellar gas from which stars can form. Stars feed energy and metal-enriched matter back into the ambient medium via stellar winds and supernovae (SNe), thereby distributing processed material throughout the ISM. Hence, through this gas-star-gas cycle, the ISM becomes a mediator of SF.
In Section 2, the different phases of the ISM in galaxies will be introduced, and Section 3 presents observations of the ISM in the Milky Way. Section 4 will show that the Galactic ISM is not confined to the thin disk, but extends to high distances (z) above the plane, forming a gaseous halo. However, being located within the disk of the Milky Way, we lack a grand overview. Therefore, many global parameters of interstellar matter can be better and more easily determined by looking at external systems. For spatial investigations of galaxy disks a face-on viewing geometry is favorable, while for investigations of the vertical structure of galaxies edge-on systems are best-suited. In the following paragraphs, I will give a summary of our observational knowledge about the ISM in both the disks (Section 5) and the halos (Section 6) of external spiral galaxies and provide examples of key observations. Section 7 summarizes the observational evidence for the complex nature of the halo ISM in external galaxies, while Section 8 provides a brief account of how theoretical models describe the properties of interstellar matter, including disk-halo interactions in both starburst and ``normal'' spiral galaxies. The goal of this article is to show that it takes a certain minimal amount of energy input - coming from SF processes - in the disks of spiral galaxies to initiate outflows creating gaseous halos. Outflow phenomena being the primary cause for the existence of gaseous halos, the halo ISM of late-type galaxies can then be understood as a natural extension of the ISM in the galaxy disks.