A Spacetime View of Entropy: From Black Holes to Holography

It was 50 years ago that Bekenstein first posited the generalized second law and the idea that a black hole carries an entropy proportional to its horizon area. Since then, we have learned that the connection between areas and entropies is not restricted to black holes, but rather is a general feature of quantum gravity appearing in many guises. Indeed, this connection extends in a sense even to non-gravitational quantum field theories and quantum many-body systems. I will review some of this history and our current, still incomplete understanding of entropy in quantum gravity, with a particular focus on the class of theories that we understand the best, namely the holographic ones, and the area-entropy connection discovered there by Ryu-Takayanagi.