  We summarize the evidence from multiwavelength observations that the dominant
power source in the majority of ultraluminous infrared galaxies (ULIGs) may be
an active galactic nucleus (AGN). In the broader context of the debate, we also
show that: 1. ULIGs are indeed a key stage in the transformation of merging
gas-rich disks into ellipticals, 2. ULIGs are plausibly the precursors of
quasi-stellar objects (QSOs), and 3. ULIGs do appear to be local templates of
the high luminosity tail of major gas-rich mergers at z ~ 1-4.
