  We study gauge coupling unification and various phenomenological issues, such
as baryon number conservation, the $\mu $ problem and neutrino anomalies,
within SUSY 5D orbifold models. The 5D MSSM on an $S^{(1)}/Z_2$ orbifold with
'minimal' field content does not lead to low scale unification, while some of
its extensions can give unification near the multi TeV scale. Within the
orbifold SU(5) GUT, low scale unification can not be realized due to full SU(5)
multiplets participating in the renormalization above the compactification
scale. As alternative examples, we construct 5D N=1 SUSY Pati-Salam $SU(4)_c\tm
SU(2)_L\tm SU(2)_R\equiv G_{422}$ and flipped $SU(5)\tm U(1)\equiv G_{51}$ GUTs
[both maximal subgroups of SO(10)] on an $S^{(1)}/Z_2\tm Z_2'$ orbifold. New
examples of low scale unifications within $G_{422}$ are presented. For $G_{51}$
the unification scale is shown to be necessarily close to $\sim 10^{16}$ GeV.
The possible influence of brane couplings on the gauge coupling unification is
also outlined. For the resolution of the various phenomenological problems
extensions with a discrete ${\cal Z}$ symmetry turn out to be very effective.
