The holographic principle, a fundamental principle in quantum gravity, asserts that the information content of spacetime regions is limited by the area of surfaces in spacetime. This principle is rooted in the quantum properties of black holes and the construction of light-sheets, which associate spacetime regions to any given spacetime. The holographic principle is expressed through the covariant entropy bound, which relates the number of quantum states of matter to the geometry of spacetime. This bound is tested and found to hold in various examples, including cosmological systems and gravitational collapse. The holographic principle challenges local theories and suggests a preferred role for null hypersurfaces in quantum gravity. It is closely related to string theory and the AdS/CFT correspondence, but its implementation in a unified theory remains an active area of research. The principle has implications for the cosmological constant problem and the nature of spacetime, suggesting that the world may be a hologram.The holographic principle, a fundamental principle in quantum gravity, asserts that the information content of spacetime regions is limited by the area of surfaces in spacetime. This principle is rooted in the quantum properties of black holes and the construction of light-sheets, which associate spacetime regions to any given spacetime. The holographic principle is expressed through the covariant entropy bound, which relates the number of quantum states of matter to the geometry of spacetime. This bound is tested and found to hold in various examples, including cosmological systems and gravitational collapse. The holographic principle challenges local theories and suggests a preferred role for null hypersurfaces in quantum gravity. It is closely related to string theory and the AdS/CFT correspondence, but its implementation in a unified theory remains an active area of research. The principle has implications for the cosmological constant problem and the nature of spacetime, suggesting that the world may be a hologram.