Far infrared fine structure lines in the interstellar medium

Fine structure lines in the far infrared possess properties that make them useful tools for diagnosing astrophysical plasmas. The line ratios are sensitive and accurate probes of density, elemental abundances, and ionization structure. Studies of line shapes and Doppler shifts should prove valuable in studies of the gross dynamics of H II regions and galaxies. In the present paper, the theory involved in the interpretation of these lines is reviewed, and available observations are summarized.