  The discrepancy between the measured Lamb shift in muonic hydrogen and
expectations from electron-proton scattering and regular hydrogen spectroscopy
has become known as the proton radius puzzle, whose most "mundane" resolution
requires a $> 5 \sigma$ shift in the value of the fundamental Rydberg constant.
I briefly review the status of spectroscopic and scattering measurements,
recent theoretical developments, and implications for fundamental physics.
