  Although wormholes can be treated as topological objects in spacetime and
from a global point-of-view, a precise definition of what a wormhole throat is
and where it can be located can be developed and treated entirely in terms of
local geometry. This has the advantage of being free from unnecesary technical
assumptions about asymptotic flatness, and other global properties of the
spacetime containing the wormhole. We discuss our recent work proving that the
violation of the null energy condition (NEC) is a generic feature of all
wormholes, whether they be time-dependent or static, and demonstrate that
time-dependent wormholes have two throats, one for each direction through the
wormhole, which coalesce only in the static limit.
