  The specification of energy for gravitating systems has been an unsettled
issue since Einstein proposed his pseudotensor. It is now understood that
energy-momentum is \emph{quasi-local} (associated with a closed 2-surface).
Here we consider quasi-local proposals (including pseudotensors) in the
Lagrangian-Noether-Hamiltonian formulations. There are two ambiguities: (i)
there are many possible expressions, (ii) they depend on some non-dynamical
structure, e.g., a reference frame. The Hamiltonian approach gives a handle on
both problems. The Hamiltonian perspective helped us to make a remarkable
discovery: with an isometric Minkowski reference a large class of
expressions---namely all those that agree with the Einstein pseudotensor's
Freud superpotential to linear order---give a common quasi-local energy value.
Moreover, with a best-matched reference on the boundary this is the Wang-Yau
mass value.
