  There appears to be three, perhaps related, ways of approaching the nature of
vacuum energy . The first is to say that it is just the lowest energy state of
a given, usually quantum, system. The second is to equate vacuum energy with
the Casimir energy. The third is to note that an energy difference from a
complete vacuum might have some long range effect, typically this energy
difference is interpreted as the cosmological constant. All three approaches
are reviewed, with an emphasis on recent work. It is hoped that this review is
comprehensive in scope. There is a discussion on whether there is a relation
between vacuum energy and inertia. The solution suggested here to the nature of
the vacuum is that Casimir energy can produce short range effects because of
boundary conditions, but that at long range there is no overall effect of
vacuum energy, unless one considers lagrangians of higher order than Einstein's
as vacuum induced. No original calculations are presented in support of this
position. This is not a review of the cosmological constant {\it per se}, but
rather vacuum energy in general, my approach to the cosmological constant is
not standard.
