The course “Physics of the Interstellar Medium” covers the basic physical processes that regulate the state of diffuse gas in, around and between galaxies. Therefore, we will extend our attention also the intergalactic medium, since this is a very vital research area in modern astrophysics.Topics covered include the chemical composition and the phases of the interstellar medium, the equilibrium of gaseous nebulae, the formation of the emission and absorption lines, the properties of the interstellar dust, the molecular gas and the process of star formation, supernova explosions, galactic superwinds.We will also discuss the observational techniques that allow us to investigate the physics of the interstellar and intergalactic media. In particular, we will address the formation of emission and absorption spectral lines (at UV, optical and infrared wavelengths). The goal is to develop the background required to understand current research on the interplay between star formation and cosmic gas. Some lectures will be dedicated to the discussion of recent papers, assigned as complementary readings.