Proton charge radius

The so-called proton charge radius puzzle was one of the challenging problems in physics in the last decade. A significant (at the level of four standard deviations (4σ)) difference between the values of the root-mean-square proton charge radius measured in normal and muonic hydrogen has kindled lively discussions among both experimentalists and theoreticians specializing in quantum electrodynamics. The problem becomes even more glaring (up to 7σ) if data on the scattering of electrons on protons are taken into account. We review various methods that enable measurement of the proton charge radius, analyze the origin of the disagreement, and present results of recent experiments that aim at resolving this puzzle.