Describing interstellar matter in our galaxy in all of its various forms, this book also considers the physical and chemical processes that are occurring within this matter. The Interstellar Medium begins by explaining the various components that comprise it and detailing the ways that we are able to study them. The following section is devoted to the physical, chemical and dynamical processes that control the behaviour of interstellar matter, especially the instabilities and cloud collapse processes that lead to the formation of stars. It concludes by summarizing the transformations that can occur between the different phases of the interstellar medium. Emphasizing methods over results, The Interstellar Medium is written for graduate students, for young astronomers, and also for any researchers who have developed an interest in the interstellar medium.