A large percentage of spiral galaxies are forming stars at a very high rate in their nuclei. There are indications that the process of star formation is modified significantly in these regions, compared with the solar neighborhood. The star formation may be fueled by interstellar material that is captured by the nucleus. Further work is needed to explore these possibilities and to establish the evolutionary connections among galactic nuclei, various types of which can be distinguished by their radically different infrared properties.

In this paper we review the observed infrared properties of the general classes of active extragalactic nuclei with the purpose of relating the observations to the mechanisms responsible for the emission processes. We will first give a summary of those observations which define the energy distributions and emission line ratios of broad groups of objects. We will intersperse measurements of specific features throughout the discussion that illustrate definite emission mechanisms.

No one could be more of an outsider to infrared astronomy than myself. Like all outsiders, one forms preconceived notions about disciplines outside one's own speciality. In my own case, I can summarise these under three headings.