The root-mean-square (rms) charge radius r p of the proton has so far been known only with a surprisingly low precision of about 1% from both electron scattering and precision spectroscopy of hydrogen. We have recently determined r p by means of laser spectroscopy of the Lamb shift in the exotic "muonic hydrogen" Published in " " which should be cited to refer to this work.http://doc.rero.ch atom. Here, the muon, which is the 200 times heavier cousin of the electron, orbits the proton with a 200 times smaller Bohr radius. This enhances the sensitivity to the proton's finite size tremendously. Our new value r p = 0.84184 (67) fm is ten times more precise than the generally accepted CODATA-value, but it differs by 5 standard deviations from it. A lively discussion about possible solutions to the "proton size puzzle" has started. Our measurement, together with precise measurements of the 1S-2S transition in regular hydrogen and deuterium, also yields improved values of the Rydberg constant, R ∞ = 10,973,731.568160 (16) m −1 .