  Energy has an ambiguous status in general relativity. For systems embedded in
asymptotically flat space-times it is possible to construct an integral
invariant that corresponds to total energy, however there is no local
differential invariant that can be identified with energy density. Moreover, in
cosmological 'big-bang' scenarios there is an energy gain of about 70 orders of
magnitude between the initial detonation and final inflation. Nevertheless,
there is a widespread belief that all physical systems, irrespective of their
size or complexity, can be associated with a unique scalar measure --- their
energy. This presumption suggests parallels with the mathematical concept of
measure in set theory as well as with entropy. However, both analogies are
limited in scope. We also discuss a wide variety of other forms of energy
ranging from particle physics to information theory, and consider the
implications for conservation laws. Finally, we recall several historical
episodes in which energy conservation was at the center of controversy.
