  Newton's gravitational constant is shown to be a running coupling constant,
much like the familiar running gauge couplings of the Standard Model. This
implies that, in models with appropriate particle content, the true Planck
scale, i.e. the scale at which quantum gravity effects become important, can
have a value different from 10^19 GeV, which would be expected from naive
dimensional analysis. Then, two scenarios involving this running effect are
presented. The first one is a model which employs huge particle content to
realize quantum gravity at the TeV scale in 4 dimensions, thereby solving the
hierarchy problem of the Standard Model. Secondly, effects of the running of
Newton's constant in grand unified theories are examined and shown to introduce
new significant uncertainties in their predictions, but possibly also to
provide better gauge coupling unification results in some cases.
