  The usual approaches to the definition of energy give an ambiguous result for
the energy of fields in the radiating regime. We show that for a massless
scalar field in Minkowski space-time the definition may be rendered unambiguous
by adding the requirement that the energy cannot increase in retarded time. We
present a similar theorem for the gravitational field, proved elsewhere, which
establishes that the Trautman-Bondi energy is the unique (up to a
multiplicative factor) functional, within a natural class, which is monotonic
in time for all solutions of the vacuum Einstein equations admitting a smooth
``piece'' of conformal null infinity Scri.
