The space between the stars contains a large diversity of objects in which physical processes occur that are fundamental to the structure and evolution of galaxies. This book offers the reader a basic knowledge of these processes and presents simple numeric estimates of the main quantities relevant to the interstellar medium. The main objects that constitute the interstellar space are described, but the emphasis of the book lies in the physical processes occurring in these objects, which may also occur in other astrophysical environments. The book is directed tor graduate as well as advanced undergraduate students of physics and astrophysics.