\indent one of the greatest accomplishments of the theory of general relativity in the past century is the proof of the positive mass/energy theorem for asymptotically flat spacetime. this provides the theoretical foundation for the stability of an isolated gravitating system. however, the concept of mass/energy remains a challenging problem because of the lack of a quasilocal description. most observable physical models are finitely extended spatial regions and measurement of mass/energy on such a region is essential in many fundamental issues. in fact, among penrose's list of major unsolved problems in classical general relativity, the first one is ``find a suitable quasi-local definition of energy-momentum in general relativity". in this talk, i shall describe a new proposal of quasi-local mass/energy by shing-tung yau of harvard university and myself.