  Using the expansions of the heavy meson decay widths in the heavy quark mass
and QCD sum rules for estimates of corresponding matrix elements,\, we
calculate the $D^{\pm,o,s}$ and $B^{\pm,o,s}$ meson lifetimes. The results for
D mesons are in a reasonable agreement with the data,\, while it is predicted:
$[\Gamma (B_d)-\Gamma (B^\pm)]/\Gamma_B\se 4\%\,$ (and the lifetime difference
of the $B_d$ and $B_s$ mesons is even smaller);\,
$[\Gamma(B_s^{short})-\Gamma(B_s^{long})]/{\ov \Gamma}(B_s)\se 8\%\,.$ The role
of the weak annihilation and Pauli interference contributions to the lifetime
differences is described in detail. In the course of self-consistent
calculations the values of many parameters crucial for calculations with
charmed and beauty mesons are found. In particular,\, the quark pole masses
are: $M_c\se 1.65\,GeV,\,\, M_b\se 5.04\,GeV\,,$ and the decay constants are:
$f_D(M_c)\se 165\,MeV\,,\,\,f_B(M_b)\se 120\,MeV\,$. It is also shown that the
nonfactorizable corrections to the $B-{\bar B}$ mixing are large,\,
$B_B(M_b)\se (1-18\%)\,.$ The values of the unitarity triangle parameters are
found which are consistent with these results and the data available (except
for the NA31 result for the $\epsilon ^{\prime}/\epsilon$ which is too large):
$|V_{cb}|\se 4.2\cdot 10^{-2}\,,\, |V_{td}|\se 1.3\cdot 10^{-2}\,,\,
|V_{ub}/V_{cb}|\se 0.10\,,\, \{\,A\se 0.86\,,\,\,\rho \se -0.40\,,\,\, \eta \se
0.20\,\}.$
