  We review the formalism by which the tunnelling probability of an unstable
ground state can be computed in quantum field theory, with special reference to
the Standard Model of electroweak interactions. We describe in some detail the
approximations implicitly adopted in such calculation. Particular attention is
devoted to the role of scale invariance, and to the different implications of
scale-invariance violations due to quantum effects and possible new degrees of
freedom. We show that new interactions characterized by a new energy scale,
close to the Planck mass, do not invalidate the main conclusions about the
stability of the Standard Model ground state derived in absence of such terms.
