Discrete symmetries play a crucial role in particle physics.
They appear abundantly in string model constructions.
We focus here on the case of discrete $R$-symmetries
which are intrinsically connected to the Lorentz group in extra dimensions
and the
appearance of $N$-extended supersymmetry. In that sense, discrete
$R$-symmetries
can be understood as "fractionally" extended supersymmetry.
These symmetries reveal insight about the
location of fields in extra dimensions (in particular the Higgs boson).
Applications can be
found in the solution of the $\mu$-problem, suppression of proton decay and
the structure
of the soft terms of broken supersymmetry.