  The interior of a planetary nebula (PN) is expected to be filled with shocked
fast wind from the central star. This hot gas plays the most important role in
the dynamical evolution of the PN; however, its physical conditions are not
well-known because useful X-ray and far-UV observations were not available
until the advent of Chandra, XMM-Newton, and FUSE. This paper reviews X-ray
observations of the hot gas in PN interiors and far-UV observations of the
interfaces between the hot gas and the dense nebular shells.
