We discuss the masses of the leptoquark bosons mx and the proton lifetime in grand unified theories based principally on SU(5). It is emphasized that estimates of mx based on the QCD coupling and the fine structure constant are probably more reliable than those using the experimental value of sin2 θw. Uncertainties in the QCD Λ parameter and the correct value of α are discussed. We estimate higher-order effects on the evolution of coupling constants in a momentum-space renormalization scheme. It is shown that increasing the number of generations of fermions beyond the minimal three increases mx by almost a factor of 2 per generation. Additional uncertainties exist for each generation of technifermions that may exist. We discuss and discount the possibility that proton decay could be "Cabibbo rotated" away, and a speculation that Lorentz invariance may be violated in proton decay at a detectable level. We estimate that in the absence of any substantial new physics beyond that in the minimal SU(5) model the proton lifetime is 8 × 1030±2 years.