  When the cosmological constant, $\Lambda$, is interpreted as a thermodynamic
variable in the study of black hole thermodynamics a very rich structure
emerges. It is natural to interpret $\Lambda$ as a pressure and define the
thermodynamically conjugate variable to be the thermodynamic volume of the
black hole (which need not bear any relation to the geometric volume). Recent
progress in this new direction for black hole thermodynamics is reviewed.
