  A dynamically preferred quasi-local definition of gravitational energy is
given in terms of the Hamiltonian of a `2+2' formulation of general relativity.
The energy is well-defined for any compact orientable spatial 2-surface, and
depends on the fundamental forms only. The energy is zero for any surface in
flat spacetime, and reduces to the Hawking mass in the absence of shear and
twist. For asymptotically flat spacetimes, the energy tends to the Bondi mass
at null infinity and the \ADM mass at spatial infinity, taking the limit along
a foliation parametrised by area radius. The energy is calculated for the
Schwarzschild, Reissner-Nordstr\"om and Robertson-Walker solutions, and for
plane waves and colliding plane waves. Energy inequalities are discussed, and
for static black holes the irreducible mass is obtained on the horizon.
Criteria for an adequate definition of quasi-local energy are discussed.
