The success of the B Factories at SLAC and KEK has advanced
our knowledge of CP violation, a prerequisite for the creation
of the matter-dominant universe.  Steady increase of data has
brought a new era of precision CP measurements, in which the
unitarity of the Cabibbo-Kobayashi-Maskawa matrix is scrutinized
for signs of New Physics.  One of the crucial information,
surprisingly, comes not from CP violation but from studies of
semileptnic decays of the B mesons.  I will discuss how various
measurements of the B semileptonic decays mesh together to
determine the size of the smallest CKM matrix element |Vub|,
and how such measurements are made possible through progress
in the theory of B decays.