  To start the $b$-decay session we briefly introduce and comment some
important theoretical tools which are currently used in $b$ physics. Heavy
Quark Symmetry and its consequences for heavy to heavy and heavy to light
semi-leptonic decays, as well as for leptonic decays, are briefly summarised.
It is stressed that symmetry must be completed with dynamical calculations. A
critical discussion of the nearest pole dominance (VMD) assumption is
performed. Parton model and its higher twist corrections are discussed on the
example of lifetimes. Finally non-leptonic decays are considered via the
example of the exclusive calculation of $\Delta \Gamma$ in the $B_s-\overline
B_s$ system. The popular factorization assumption is discussed and seems to be
rather good.
