  We consider proton decay in theories that contain large extra dimensions. If
virtual black hole states are allowed by the theory, as is generally the case,
then proton decay can proceed via virtual black holes. The experimental limits
on the proton lifetime place strong constraints on the quantum gravity scale
$\mnew$ (the effective Planck mass). For most theories, our constraint implies
a lower bound of $\mnew > 10^{16}$ GeV. The corresponding bound on the size of
large extra dimensions is $\ell < 10^{6/n} \times 10^{-30}$ cm, where $n$ is
the number of such dimensions. Regrettably, for most theories this limit rules
out the possibility of observing large extra dimensions at accelerators or in
millimeter scale gravity experiments. Conversely, proton decay could be
dominated by virtual black holes, providing an experimental probe to study
stringy quantum gravity physics.
