New work has shown how to complete the correspondence between the physics of black holes and the laws of thermodynamics by incorporating volume and pressure into the formalism. This results in a change in the interpretation of the black hole's mass, and may give a new handle on understanding aspects of theories of gravity with non-vanishing cosmological constant. This subject also serves as a surprising new source of novel equations of state with which to build thermodynamic systems! This talk will try to motivate and explain some of these developments.