Inteller space is the hardest vacuum that we know of. But it is not empty. Over the vast distances there are atoms, molecules, dust grains, radiations and more. It is a coprehensive overview of our current theories as backed up by the most recent observational understanding of the interstellar medium of galaxies as of 2005. As this is a rapidly developing field, the book is filled with references for the student to find more information from web sites and the like that will contain the most recent findings, research, and theories. This book is suitable for a course for advanced undergraduate and graduate students, and for research scientists. The author is a professor of Astrophysics at the Kapteyn Astronomical Institute in the Netherlands and is a senior scientist with the Dutch Space Agency, SRON.