  [Talk presented at the International Seminar Quarks `92, Zvenigorod, Russia,
May 11-17, 1992.] The electroweak vacuum need not be absolutely stable. For
certain top and Higgs masses in the Minimal Standard Model, it is instead
metastable with a lifetime exceeding the present age of the Universe. The decay
of our vacuum may be nucleated at low temperature by quantum tunneling or at
high temperature in the early Universe by thermal excitation. I briefly review
the constraints on top and Higgs masses from requiring that the electroweak
vacuum be sufficiently stable to have survived to the present day.

High Energy Physics - Phenomenology