Ten years have passed since It Hooft and Polyakov demonstrat- ed that superheavy magnetic monopoles were a natural consequence of any Grand Unified Theory (GUT) in which the unifying group contains a U(l) factor as a subgroup. An analysis of these GUTs in an expanding, cooling universe yields a phase transition at an energy l015 GeV and at a cosmic time lO-35 seconds after the big bang. The general consequences of GUTs and this phase transition are the prediction of proton decay, the production of superheavy magnetic monopoles, and an understanding of the observed excess of matter over...