One of the most notable results of the IRAS mission was the recognition of a class of galaxies with large bolometric luminosities, most of whose radiation emerges in the far-infrared (typically 50-100μ). The space density of these “IRAS galaxies” approaches that of QSOs for comparable luminosities, so that these are a significant component of the population of galaxies. The frequent occurrence of interacting and merging galaxies among these infrared-bright systems has underscored the importance of understanding how various energetic processes (active nuclei, starbursts) are related to the disturbances produced during galaxy encounters. The same material directly responsible for the signature infrared excess of these systems degrades many of the diagnostics normally useful for probing the nature of he observed activity, so that in many systems it remains unclear just what kinds of phenomena are being observed.