  We present phenomenological aspects of the simplest realistic SU(5) grand
unified theory--the theory with the 5, 15, and 24 dimensional representations
in the Higgs sector. We show that a successful gauge coupling unification sets
experimentally accessible upper bound on the total proton decay lifetime. It
also relates proton decay lifetime to scalar leptoquark mass in an
experimentally testable manner. We also discuss an addition of gauge
singlets--both fermions and bosons--to the simplest scenario and comment on
relevant phenomenological consequences of such modifications.
