  The exciton binding energy ($E_b$) is a key parameter that governs the
physics of many optoelectronic devices. At their best, trustworthy and precise
measurements of $E_b$ challenge theoreticians to refine models, are a driving
force in advancing the understanding a material system, and lead to efficient
device design. At their worst, inaccurate $E_b$ measurements lead theoreticians
astray, sew confusion within the research community, and hinder device
improvements by leading to poor designs. This review article seeks to highlight
the pros and cons of different measurement techniques used to determine $E_b$,
namely, temperature-dependent photoluminescence, resolving Rydberg states,
electroabsorption, magnetoabsorption, scanning tunneling spectroscopy, and
fitting the optical absorption. Due to numerous conflicting $E_b$ values
reported for halide perovskites (HP) and transition metal dichalcogenides
(TMDC) monolayers, an emphasis is placed on highlighting these measurements in
attempt to reconcile the variance between different measurement techniques. By
considering the published data en masse, we argue the experiments with the
clearest indicators are in agreement on the following values: ~350 - 450 meV
for TMDC monolayers between SiO$_2$ and vacuum, ~150 - 200 meV for
hBN-encapsulated TMDC monolayers, ~200 - 300 meV for common lead-iodide 2D HPs,
and ~10 meV for methylammonium lead iodide.
