  Quantum field theory allows for the suppression of vacuum fluctuations,
leading to sub-vacuum phenomena. One of these is the appearance of local
negative energy density. Selected aspects of negative energy will be reviewed,
including the quantum inequalities which limit its magnitude and duration.
However, these inequalities allow the possibility that negative energy and
related effects might be observable. Some recent proposals for experiments to
search for sub-vacuum phenomena will be discussed. Fluctuations of the energy
density around its mean value will also be considered, and some recent results
on a probability distribution for the energy density in two dimensional
spacetime are summarized.

Quantum Physics;General Relativity and Quantum Cosmology;High Energy Physics - Theory