 These facts raise several fundamental puzzles about the nature of entropy as a physical quantity and the relations among the different theories that involve it. How can such a quantity, which hitherto has been attributed only to material systems such as fluids and Maxwell fields, be attributed to simple regions of spacetime itself? How does general gravitational entropy relate to more standard forms of entropy, and how may the nature of general gravitational entropy itself inform our understanding of the standard forms? Does it shed new light on traditional general philosophical topics of interest, such as questions about reduction and emergence of thermodynamics to and from statistical mechanics? 