  A model independent approach based on a generalization of the operator
product expansion is used to describe semileptonic decays of heavy flavors. In
the first part of the dissertation we calculate differential distributions in
the inclusive semileptonic weak decays of heavy flavors in QCD. In particular,
the double distribution in electron energy and invariant mass of the lepton
pair is calculated. The distributions are calculated as series in $m_Q^{-1}$
where $m_Q$ is the heavy quark mass. All effects up to $m_Q^{-2}$ are included.
  Also calculated are the energy distribution and semileptonic decay width for
the case of a heavy lepton in the final state.
  Based on the results of the first part of the work, we calculate the full up
to order $1/m_Q^2$ set of the OPE sum rules for the heavy flavor transitions
and radiative corrections to them up to order $\alpha_s\Lambda_{QCD}/m_Q$.
  In the second part of this work a new model is proposed for the inclusive
semileptonic decays of the $B$ mesons $B\rightarrow l\bar{\nu}X_c$, which is
{\em defined} by the requirement that it should satisfy the QCD consistency
conditions. These conditions are imposed in the form of the OPE sum rules. It
is shown that under some natural assumptions the OPE sum rules provide
sufficient number of constraints to fully determine exclusive contributions of
a number of resonances in the final state of the decay. The theoretical results
for the model are presented in the form suitable for comparison with
experimental data. The proposed model can be used for experimental measurements
of the heavy meson matrix element $\mu _\pi ^2$ which describes the kinetic
energy of the heavy quark inside the hadron, as well as for extraction of the
value of $|V_{cb}|$ from experimental data and other applications.
