  These lectures review some of the progress made in the quantitative
understanding of B decays. The emphasis here is on applications of QCD using
perturbative and non-perturbative techniques. In some cases, however,
phenomenological models must at present be invoked to make meaningful
comparison with data. The resulting picture is consistent with the standard
model (SM) and this agreement is quantified in terms of the branching ratios,
mixing probabilities, and lifetimes which measure the charge current and
effective flavour changing neutral current transitions involving B hadrons.
This, in turn, enables a determination of five of the nine elements of the
quark mixing matrix. We discuss several proposals on improving the precision on
the parameters of this matrix in forthcoming experiments. Issues intimately
related to the quark mixing matrix such as the profile of the unitarity
triangle and CP-violating asymmetries in B decays are discussed. In particular,
we emphasize the role of rare B decays and B^0 - $\overline{B^0}$ mixings in
testing the SM quantitatively and in searching for physics beyond the SM.
