The weak nonmesonic decay of Lambda hypernuclei gives information on t
he strangeness changing weak baryon baryon interaction. The parity vio
lation of the process has been studied by observing weak nonmesonic de
cay of polarized Lambda hypernuclei. Study of the inverse process pn -
-> p Lambda gives much cleaner information on the process. Recent prog
ress on the experimental study is presented.