  We investigate constraints that the requirements of perturbativity and gauge
coupling unification impose on extensions of the Standard Model and of the
MSSM. In particular, we discuss the renormalization group running in several
SUSY left-right symmetric and Pati-Salam models and show how the various scales
appearing in these models have to be chosen in order to achieve unification. We
find that unification in the considered models occurs typically at scales below
M^{min}_{B violation} = 10^16 GeV, implying potential conflicts with the
non-observation of proton decay. We emphasize that extending the particle
content of a model in order to push the GUT scale higher or to achieve
unification in the first place will very often lead to non-perturbative
evolution. We generalize this observation to arbitrary extensions of the
Standard Model and of the MSSM and show that the requirement of perturbativity
up to M^{min}_{B violation}, if considered a valid guideline for model
building, severely limits the particle content of any such model, especially in
the supersymmetric case. However, we also discuss several mechanisms to
circumvent perturbativity and proton decay issues, for example in certain
classes of extra dimensional models.
