The space between stars within a galaxy is not empty. Although the number density of atoms is much closer to a vacuum than what we experience in the terrestrial environment, various physical processes take place in this interstellar medium (ISM). The ISM comprises hot plasma (corona), neutral atomic gas, and molecular gas phases. Each phase corresponds to a different type of astrophysical process and is observed in a distinct wavelength regime. Star formation is the most significant phenomenon that occurs in the ISM. However, its theoretical treatment in the context of galaxy evolution remains incomplete, requiring reliance on some empirical relations. Another essential component of the ISM is dust, consisting of tiny metallic particles. Dust plays several roles, including electromagnetic interactions, heating and cooling, and catalyzing chemical reactions within the ISM. This abstract touches on a few selected topics and phenomena that are particularly relevant to the evolution of galaxies.