The interstellar medium (or ISM) is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. This book presents topical research in the study of interstellar medium, including explosive processes in the interstellar medium as sources of ultrahigh-energy cosmic rays; heteronuclear diatomics in diffuse and translucent clouds; far ultraviolet observations of the Large Magellanic Cloud; MDH simulations of Parker instability undergoing cosmic-ray diffusion; deuterium in the interstellar medium and interaction of planetary nebulae, Eta-Carinae and supernova remnants with the interstellar medium.