  Ultraluminous galaxies in the local universe (z$\leq$0.2) emit the bulk of
their energy in the mid and far-infrared. The multiwavelength approach to these
objects has shown that they are advanced mergers of gas-rich spiral galaxies.
Galaxy-galaxy collisions took place on all cosmological time-scales, and nearby
mergers serve as local analogs to gain insight into the physical processes that
lead to the formation and trans-formation of galaxies in the more distant
universe. Here I review multiwavelength observations --with particular emphasis
on recent results obtained with ISO-- of mergers of massive galaxies driving
the formation of: 1) luminous infrared galaxies, 2) elliptical galaxy cores, 3)
luminous dust-enshrouded extranuclear starbursts, 4) symbiotic galaxies that
host AGNs, and 5) tidal dwarf galaxies. The most important implication for
studies on the formation of galaxies at early cosmological timescales is that
the distant analogs to the local ultraluminous infrared galaxies are invisible
in the ultraviolet and optical wavelength rest-frames and should be detected as
sub-millimeter sources with no optical counterparts.
