Weak matrix elements from lattice QCD

This is a critical overview of the current status of the computation of the weak hadronic matrix elements involving kaons and pions on the lattice. A special attention has been devoted to the computation of the B-K parameter, for which the world average remains B-K = 0.87(6)(13). The second (dominant) error is the uncertainty due to the use of the quenched approximation. Most obstacles in computing the K --> pipi matrix elements are also related to the use of quenched approximation. The main message is: to make a significant progress in computing the weak matrix elements on the lattice, the unquenching is necessary.