  In this review, the $X^{0}-\bar{X}^{0}$\ mixing ($%
X^{0}=B^{0},B_{s}^{0},K^{0}$) and its implication for CP\ violation in the
standard model are discussed. Both direct and mixing induced CP\ violation for
$K^{0}(\bar{K}^{0})$, $B^{0}(\bar{B}^{0})$\ and $B_{s}^{0}(% \bar{B}_{s}^{0})$\
are reviewed.
