HI observations are an excellent tool to probe the conditions of the ISM, giving information on the distribution and velocity structure of the cool and warm atomic gas in galaxies, and how that is affected by violent star formation. Currently, more distant galaxies have come within reach, allowing HI studies of galaxies of different Hubble types. In this talk I will review some of the work on the nearest galaxies and draw comparisons with our own. I will use the example of Holmberg II, a dwarf galaxy, to show how much can be learnt regarding violent star formation and its effects on the ISM by going to low-mass systems. Lastly, I will introduce some of the new and exciting projects which are under way.

Introduction

Carl Heiles' pioneering work on the structure of the Interstellar Medium (ISM) of our Galaxy confirmed the picture of it being a violent environment. His HI observations showed a wealth of structure in the form of shells and supershells, many of them expanding (Heiles 1979, 1984). His observations agreed, in general, with the prediction by Cox and Smith (1974) who, argued that supernovae create cavities of hot, coronal gas and play a significant rôle in the shaping of the ISM. Although some work was done on the Magellanic Clouds, it wasn't until the mid-80's that Heiles' results were dramatically confirmed by high-resolution (aperture synthesis) HI maps of the nearest galaxies, M31 and M33, enabling studies of the distribution and characteristics of HI shells across entire galaxies.