Recent results from lattice QCD simulations provide a realistic picture, based upon first principles, of~$Upsilon$ physics. We combine these results with the experimentally measured mass of the $Upsilon$~meson to obtain an accurate and reliable value for the $b$-quarks pole mass. We use two different methods, each of which yields a mass consistent with $M_b = 5.0(2)$~GeV. This corresponds to a bare mass of $M_b^0 = 4.0(1)$~GeV in our lattice theory and an $msbar$~mass of $M_b^msbar(M_b)=4.0(1)$~GeV. We discuss the implications of this result for the $c$-quark mass. ******************************************************************************* THIS IS THE VERSION WHICH WILL BE PUBLISHED IN PRL. SUBSTANTIAL MATERIAL HAS BEEN ADDED, INCLUDING RESULTS WITH DYNAMICAL FERMIONS AND A CALCULATION OF THE MSBAR MASS. *******************************************************************************
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