  In the past several years, observational entropy has been developed as both a
(time-dependent) quantum generalization of Boltzmann entropy, and as a rather
general framework to encompass classical and quantum equilibrium and
non-equilibrium coarse-grained entropy. In this paper we review the
construction, interpretation, most important properties, and some applications
of this framework. The treatment is self-contained and relatively pedagogical,
aimed at a broad class of researchers.
