There is strong evidence that the area of any surface limits the information content of adjacent
spacetime regions, at 1.431069 bits per square meter. This article reviews 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. This article explains how the bound is tested, and its validity is
demonstrated 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. This article surveys some
successes and challenges in implementing the holographic principle.