  It is noted that, but for one missing piece -- proton decay -- the evidence
in support of grand unification is now strong. It includes: (i) the observed
family-structure, (ii) the meeting of the gauge couplings, (iii)
neutrino-oscillations, (iv) the intricate pattern of the masses and mixings of
all fermions, including the neutrinos, and (v) the need for $B-L$ as a
generator, to implement baryogenesis. Taken together, these not only favor
grand unification but in fact select out a particular route to such
unification, based on the ideas of supersymmetry, SU(4)-color and left-right
symmetry. Thus they point to the relevance of an effective string-unified
G(224) or SO(10)-symmetry.
  A concrete proposal is presented, within a predictive
SO(10)/G(224)-framework, that successfully describes the masses and mixings of
all fermions, including the neutrinos - with eight predictions, all in
agreement with observation. Within this framework, a systematic study of proton
decay is carried out, which pays special attention to its dependence on the
fermion masses, including the superheavy Majorana masses of the right-handed
neutrinos. The study shows that a conservative upper limit on the proton
lifetime is about (1/2 - 1)$\times10^{34}$ yrs, with $\overline{\nu}K^{+}$
being the dominant decay mode, and as a distinctive feature, $\mu^{+}K^{0}$
being prominent. This in turn strongly suggests that an improvement in the
current sensitivity by a factor of five to ten (compared to SuperK) ought to
reveal proton decay. Otherwise some promising and remarkably successful ideas
on unification would suffer a major setback.
