Models of the nonmesonic weak decay

I review the current status of understanding the mechanism of the nonmesonic weak decays (NMWD) of hypernuclei. Long standing problem on the Γnn/Γpn ratio has been solved by considering short-range weak interactions properly. This leaves a few remaining problems, such as the asymmetry of the emitted proton with respect to the Λ polarization and validity of the ΔI=1/2 in the nonmesonic weak decay.