Abstract: Quantum extremal surfaces (QESs) play an important role in the study of the black hole information problem, and, in a sense, are semi-classical generalizations of black hole horizons. Their thermodynamics, however, have largely been unexplored because a proper interpretation requires a detailed understanding of the backreaction of quantum fields outside of a black hole. In this talk I will consider massless quantum fields outside of an eternal anti-de Sitter (AdS) black hole in two-dimensional Jackiw-Teitelboim (JT) gravity, where back reaction may be studied exactly. Accounting for backreaction, I will show the gravitational (Wald) entropy is equivalent to the generalized entropy, whose extremization leads to a QES just outside of the eternal black hole horizon. I will then derive a first law of QES thermodynamics, where the choice of thermal ensemble plays an important role. Lastly,  I will describe how QES thermodynamics relates to the Page curve of an eternal AdS_{2} black hole, and hint at a potentially new derivation of the "island formula" using a microcanonical path integral.