Very recently, the QCD group (``MILC Collaboration'') has been able to control the last remaining uncertainty in the evaluation of the ``leptonic decay constants'' of B mesons. These constants are crucial for the interpretation of experiments at Fermilab, CLEO (Cornell), and the soon-to-be-completed ``B factories'' at SLAC (Stanford) and KEK (Japan). For the first time, MILC has computed the effects of virtual quark-antiquark pairs on the decay constants. This results in large increases (of about 30%) in the predicted values. The virtual quark-antiquark effects had been ignored in most previous computations; in MILC's earlier work, the effects were estimated but not computed. For a detailed status report, see the talk by C. Bernard at the Aspen Winter Conference on Partilce Physics: here.