  We trace the origins and development of black hole thermodynamics across the
past half-century, emphasizing the framework's relation to classical
thermodynamics, and the vital role played by the notions of equilibrium,
stationarity, and symmetry. We discuss different interpretations of the first
law of black hole mechanics, and assess the validity of its mechanical,
process-based interpretation for evaporating black holes. We bring these ideas
to the cosmological realm, and highlight the various difficulties that arise
when formulating thermodynamics for black holes in asymptotically de Sitter
backgrounds. We discuss a number of proposed solutions and the open questions
that arise therein.
