  We study gauge coupling unification in the presence of extra dimensions
compactified at a few TeV. Achieving unification requires a large number of
gauge boson Kaluza-Klein excitations lighter than the string scale, such that
the higher-dimensional gauge couplings are O(1) in string scale units.
Corrections to the gauge couplings from two or more loops are about 10% or
larger, hence string (or M) theory is generally expected to be strongly coupled
in TeV-scale extra-dimensional scenarios. Higher-dimensional operators induced
by quantum gravitational effects can shift the gauge couplings by a few
percent. These effects are sufficiently large that even the minimal Standard
Model, or the MSSM, allow unification at a scale in the TeV range. The strongly
coupled unified theory may induce dynamical electroweak symmetry breaking.
