  There is strong evidence that the area of any surface limits the information
content of adjacent spacetime regions, at 10^(69) bits per square meter. We
review the developments that have led to the recognition of this entropy bound,
placing special emphasis on the quantum properties of black holes. The
construction of light-sheets, which associate relevant spacetime regions to any
given surface, is discussed in detail. We explain how the bound is tested and
demonstrate its validity in a wide range of examples.
  A universal relation between geometry and information is thus uncovered. It
has yet to be explained. The holographic principle asserts that its origin must
lie in the number of fundamental degrees of freedom involved in a unified
description of spacetime and matter. It must be manifest in an underlying
quantum theory of gravity. We survey some successes and challenges in
implementing the holographic principle.
