Gauge Coupling Unification in GUT and String Models

The results for the running of the gauge couplings in the MSSM are up-dated by proper inclusion of all low scale effects. They are presented as predictions for the strong coupling constant in the scenario with only two parameters at the GUT scale ~ (${\alpha }_U$ ~and ~$M_U$) ~and as a mismatch of the couplings at the scales ~$\sim 3\times {10}^{16}$ GeV ~and ~$4\times {10}^{17}$ GeV, when all three couplings are taken as the experimental input.