  We show that more than two generations of quarks and leptons are required to
have an anomaly free discrete R-symmetry larger than R-parity, provided that
the supersymmetric Standard Model can be minimally embedded into a grand
unified theory. This connects an explanation for the number of generations with
seemingly unrelated problems like supersymmetry breaking, proton decay, the mu
problem, and the cosmological constant through a discrete R-symmetry. We also
show that three generations is uniquely required by a non-anomalous discrete
R-symmetry in classes of grand unified theories such as the ones based on
(semi-)simple gauge groups.
