  We provide a background-independent formulation of the holographic principle.
It permits the construction of embedded hypersurfaces (screens) on which the
entire bulk information can be stored at a density of no more than one bit per
Planck area. Screens are constructed explicitly for AdS, Minkowski, and de
Sitter spaces with and without black holes, and for cosmological solutions. The
properties of screens provide clues about the character of a manifestly
holographic theory.
