The Ryu-Takayanagi formula (2006) states that the entanglement entropy of a boundary region A in a holographic CFT equals S(A) = Area(γ_A) / 4G_N, where γ_A is the minimal surface in the bulk AdS spacetime that is homologous to A. This remarkable formula connects quantum information on the boundary to classical geometry in the bulk. For a single interval of length ℓ in a 2D CFT, S = (c/3) log(ℓ/ε), matching the geometric result. The island formula (Almheiri et al. 2019) extends this to include islands in the bulk, resolving the Page curve of an evaporating black hole.