Far-infrared spectroscopy obtained with ISO's Long Wavelength Spectrometer (LWS) probes the global conditions in the ionized, molecular, and dust components of the interstellar medium in galaxies. Indeed, the great diversity in the FIR spectra of galaxies is indicative of varying gas and dust content, location, and excitation along the line of sight. Far-infrared spectroscopy is not a sensitive probe of the hard radiation fields characteristic of AGN. Instead, the far-infrared spectra of galaxies are often dominated by emission from the strongest cooling lines (i.e. [CII] 158 µm and [OI]63,146 µm) of the photodissociation regions (PDRs) from which the far-infrared continuum emission is thought to arise and by the forbidden fine-structure lines from H II regions. Because it is much less affected by extinction due to dust than spectroscopy at shorter wavelengths, FIR spectroscopy can be an important tool in probing the starburst properties of the highly obscured regions often inferred to be present in infrared-bright galaxies.
In the first part of this review, I will describe the results of far-infrared spectroscopic surveys of a variety of morphological types of galaxies, including dwarf, normal, starburst, and ultraluminous galaxies (ULIGs). The far-infrared spectra of active galactic nuclei are discussed by Spinoglio et al. elsewhere in these proceedings. I will then discuss the still uncertain physical conditions and evolutionary effects responsible for the weak emission line galaxies, many of which are ULIGs. Where possible I will discuss cross-instrument studies and compare the results of mid- and far-infrared spectroscopic surveys.