Our purpose in writing this book is to show how physics has been applied to developing our current understanding of the phase structure, physical condi- tions, chemical makeup and, evolution of the (thermal) interstellar medium. We hope it provides an up-to-date overview which postgraduates, advanced undergraduates, and professionals in astrophysics can use as a "reference of first resort" before going on to read the more specialist monographs or research literature. We have covered the exciting observational results, but without consideration of the experimental techniques or instruments...