  We investigate the prospects to achieve unification of the gauge couplings in
models without supersymmetry. We restrict our discussion to $SU(5), SO(10)$ and
$E_6$ models that mimic the structure of the Standard Model as much as possible
("conservative models"). One possible reason for the non-unification of the
standard model gauge couplings are threshold corrections which are necessary
when the masses of the superheavy fields are not exactly degenerate. We
calculate the threshold corrections in conservative models with a Grand Desert
between the electroweak and the unification scale. We argue that only in
conservative $E_6$ models the corrections can be sufficiently large to explain
the mismatch and, at the same time, yield a long-enough proton lifetime. A
second possible reason for the mismatch are particles at an intermediate scale.
We therefore also study systematically the impact of additional light scalars,
gauge bosons and fermions on the running of the gauge coupling. We argue that
for each of these possibilities there is a viable scenario with just one
intermediate scale.
