  We discuss the concepts of energy and mass in relativity. On a finitely
extended spatial region, they lead to the notion of quasilocal energy/mass for
the boundary 2-surface in spacetime. A new definition was found in [27] that
satisfies the positivity, rigidity, and asymptotics properties. The definition
makes use of the surface Hamiltonian term which arises from Hamilton-Jacobi
analysis of the gravitation action. The reference surface Hamiltonian is
associated with an isometric embedding of the 2-surface into the Minkowski
space. We discuss this new definition of mass as well as the reference sur-
face Hamiltonian. Most of the discussion is based on joint work with PoNing
Chen and Shing-Tung Yau.
