  Grand unified models in four dimensions typically suffer from the
doublet-triplet splitting problem. This obstacle can be overcome in
higher-dimensional settings, where a non-trivial gauge group topography can
explain the simultaneous appearance of complete standard model generations in
the form of 16-plets of SO(10) and the Higgs fields as split multiplets. In
these notes, the emerging scheme of `local grand unification' and its
realization in the context of orbifold compactifications of the heterotic
string are reviewed.
