Ktoπνbarν and High Precision Determinations of the CKM Matrix

We investigate the future determination of the CKM matrix using theoretically clean quantities, like $B(K^+\to\pi^+\nu\bar\nu)$, $B(K_L\to\pi^0\nu\bar\nu)$ or $\sin 2\beta$, $\sin 2\alpha$ as extracted from CP violation studies in $B$ physics. The theoretical status of $K\to\pi\nu\bar\nu$ is briefly reviewed and their phenomenological potential is compared with that of CP asymmetries in $B$ decays. We stress the unique opportunities provided by measuring the CP violating rare decay $K_L\to\pi^0\nu\bar\nu$. It is pointed out that this mode is likely to offer the most precise determination of ${\rm Im}V^*_{ts}V_{td}$ and the Jarlskog parameter $J_{CP}$, the invariant measure of CP violation in the Standard Model.