  In this thesis we present an introduction to Soft-Collinear Effective Theory,
which can be used to prove (or disprove) factorization theorems to all orders
in the strong coupling constant for some B decays into light and energetic
particles. Specifically, the factorizable amplitudes for inclusive B->X_u l nu
and exclusive B->gamma l nu are calculated in renormalization-group improved
perturbation theory to first non-trivial order. Form factors encoding the
exclusive decay amplitudes for B->P l nu and B->V l nu (P=light pseudoscalar
meson, V=light vector meson) are studied and proved to be dominated by the
non-factorizable Feynman mechanism.
