Interstellar and Intergalactic Medium is a text aimed at graduate students and advanced undergraduates in astronomy and physics; its scope is appropriate to a one-semester course. The text discusses the phases of the interstellar gas, emphasizing the role of heating, cooling, ionization, recombination, and radiative transport. The interaction between interstellar gas and dust is also discussed. In addition to the `classical' study of the interstellar medium, the rapidly developing field of circumgalactic and intergalactic gas is also included. In this text, first in the Ohio State Graduate Astrophysics Series, emphasis is placed on order-of-magnitude calculations and the development of physical insight.