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 progresses in the theory of B decays.