Patent claims define the scope of the patent right and hence are central to the operation of the patent system. Following the Supreme Court’s decision in Markman v. Westview Instruments (517 U.S. 370 (1996)), holding that “the construction of a patent, including terms of art within its claim, is exclusively within the province of the court,” district judges began the practice of construing patent claims in advance of trial following so-called “Markman” hearings. These constructions became subject to appellate review after the trial or summary judgment ruling. This chapter surveys empirical studies examining: (1) reversal rates; (2) the sources and methodologies that judges employ in construing patent claims; and (3) appellate behavior generally. It examines the hypotheses underlying these studies, the data used, the empirical methods deployed, and the principal results. It also suggests directions for further research.