Interstellar and intergalactic gas in the far IR and submillimeter spectral ranges

We discuss key physical problems related to the interstellar and intergalactic medium, which are the subject of far infrared (FIR) and submillimeter (submm) astronomy. We outline the most impressive results of the last 25 years and summarize the characteristics of the most successful space and ground-based FIR and submm observatories: those currently in operation, already inoperative, or under development. A brief discussion is given of those physical problems concerning the evolution of the gaseous medium in galaxies, their vicinity, and the intergalactic medium, whose solution can rely on critically important and sometimes otherwise inaccessible information from the FIR and submm ranges. Among such problems are the origin of the galactic molecular gas, the conversion of gas into stars, the ejection of gas and dust into intergalactic space, the long-term survival of dust in the hostile environment of this space (in particular, in the hot environment of galaxy clusters), and the origin of dust in the Universe. Recent astronomical observations give rise to a variety of surprising findings related to our understanding of the evolution of galaxies, the chemical history of the Universe, etc. A brief discussion of some of these findings is given.