  These notes provide a brief primer on the basic aspects of "observational
entropy" (also known as "quantum coarse-grained entropy"), a general framework
for applying the concept of coarse-graining to quantum systems. We review the
basic formalism, survey applications to thermodynamics, make a connection to
quantum correlations and entanglement entropy, compare to the corresponding
classical theory, and discuss a generalization based on POVM measurements.
