  After a brief review of the motivations for grand unification, I discuss the
main challenges facing realistic SUSY GUT model building. Achieving
doublet-triplet splitting without fine--tuning is chief among them. Symmetry
breaking should occur consistently without unwanted Goldston bosons, \mu-term
of order TeV for the MSSM Higgs fileds should emerge naturally, and realistic
fermion masses with small quark mixing angles and large lepton mixing angles
should be generated with some predictivity. Significant progress has been made
over the years towards achieving these goals in the context of supersymmetric
SO(10) GUT. A complete SO(10) model is presented along this line wherein,
somewhat surprisingly, the GUT scale threshold corrections to the gauge
couplings are found to be small. This results in a predictive scenario for
proton lifetime. An interesting correlation between the d=6 (p\to e^+\pi^0) and
d=5 (p\to \nu-bar K^+) decay amplitudes is observed. This class of models
predicts that both proton decay modes should be observable with an improvement
in the current sensitivity by about a factor of five to ten.
