  The effects of extra dimensions on gauge coupling unification is studied. We
start with a comparison between power law running of the gauge couplings in
models with extra dimensions and logarithmic running that happens in many
realistic cases. We then discuss the effect of extra dimensions on various
classes of unification models. We identify products of evolution coefficients
that dictate the profile of unification in different models. We use them to
study under what conditions unification of couplings can occur in both one and
two step unification models. We find that Kaluza-Klein modes can help generate
interesting intermediate scale models with gauge coupling unification such as
the minimal left-right models with the seesaw mechanism with a $M_{W_R}\sim
10^{13}$ GeV intermediate scale, useful in understanding neutrino oscillations.
We also obtain several examples where the presence of noncanonical
normalization of couplings enables us to obtain unification scales around
$10^{11}$ GeV. This fits very well into a class of models proposed recently
where the string scale is advocated to be at this value from physical
arguments.
