  We re-examine the evidence for the existence of ultra-massive (M > 1.1 M_sun)
white dwarfs based on gravitational redshift of white dwarfs in common proper
motion binaries or in clusters, on parallax measurements, on orbital solutions,
and, finally, on the analysis of hydrogen line profiles. We conclude that the
best evidence is largely based on the analysis of Balmer line profiles although
the companion to the A8V star HR 8210 is a compelling case made initially using
the large binary mass function and confirmed by an analysis of the Lyman line
spectrum. The confirmation and identification of high-mass white dwarfs, more
particularly non-DA white dwarfs, using parallax measurements may prove
critical in establishing the population fraction of these objects and in
constraining the high-end of empirical initial-mass to final-mass relations.
The existence of a substantial population of ultra-massive white dwarfs
supports the concept of a steeper initial-mass to final-mass relations linking
6 M_sun progenitors with approximately greater than 1.1 M_sun white dwarfs.
