  In light of null results from New Physics searches at the LHC, we look at
unification of the gauge couplings as a model-building principle. As a first
step, we consider extensions of the Standard Model with vector-like fermions.
We present a comprehensive list of spectra that feature fermions in two
distinct $SU(3)_C\times SU(2)_L\times U(1)_Y$ representations, in which precise
gauge coupling unification is achieved. We derive upper and lower limits on
vector-like masses from proton decay measurements, running of the strong gauge
coupling, heavy stable charged particle searches, and electroweak precision
tests. We demonstrate that due to a particular hierarchy among the mass
parameters required by the unification condition, complementarity of various
experimental strategies allows us to probe many of the successful scenarios up
to at least 10 TeV.
