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This concise textbook, the first volume in the Ohio State Astrophysics Series, covers all aspects of the interstellar and intergalactic medium for graduate students and advanced undergraduates. This series aims to impart the essential knowledge on a topic that every astrophysics graduate student should know, without going into encyclopedic depth. This text includes a full discussion of the circumgalactic medium, which bridges the space between the interstellar and intergalactic gas, and the hot intracluster gas that fills clusters of galaxies. Its breadth of coverage is innovative, as most current textbooks treat the interstellar medium in isolation. The authors emphasise an order-of-magnitude understanding of the physical processes that heat and cool the low-density gas in the universe, as well as the processes of ionization, recombination, and molecule formation. Problems at the end of each chapter are supplemented by online projects, data sets and other resources.