  We make a detailed analysis of gauge coupling unification in supersymmetry.
When the Standard Model gauge group is embedded in a Grand Unified Theory, new
particles often appear below the GUT scale in order to predict the right
phenomenology at low energy. While these new particles are beyond the reach of
accelerator experiments, they change the prediction of \alpha_s. Here we
classify all the (SU(3)_C,SU(2)_L,U(1)_Y) representations which improve or
worsen the prediction. Running experimentally determined values of the coupling
constants at two loops we calculate the allowed range of masses of fields in
these representations. We explore the implication of these results in SU(5) and
SU(3)^3 (trinification) models. We discover that minimal trinification predicts
light triplet Higgs particles which lead to proton decay with a lifetime in the
vicinity of the current experimental bound.
