  Two body weak processes $\Lambda N \leftrightarrow NN$ are studied from the
viewpoint of quark substructure of baryons.They can be studied in nonmesonic
weak decays of hypernuclei and also hyperon production in the $NN$ scattering.
The direct quark mechanism in contrast to the one-pion exchange mechanism is
shown to give sizable $\di=3/2$ contribution in the J=0 channel. Nonmesonic
decay rates of $^4_{\Lambda}$H, $^4_{\Lambda}$He, and $^5_{\Lambda}$He agree
with the current experimental data fairly well. Especially the $n/p$ ratio is
enhanced by the direct quark contribution. The cross sections for $pn\to \Lam
p$ scattering are studied in the quark cluster model approach.
