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 Gamma_n/Gamma_p 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 Lambda polarization and validity of the Delta I=1/2 in the nonmesonic weak decay.