The proton radius was recently measured in muonic hydrogen spectroscopy giving a results that is 7-sigma (4.4%) away from its CODATA value. This value is based on measurements which come both from hydrogen spectroscopy and from the electron-proton scattering. Muonic hydrogen is around 8-million times more sensitive to the effects of the finite size of the proton than hydrogen, since the muon is around 200 times heavier than the electron, which makes this discrepancy one of the most interesting puzzles in physics nowadays. We will review how the proton radius is defined and which are the effects that are measured in the experiments. The effect of the proton radius in spectroscopy is measured from the Lamb shift, the splitting between 2P-2S energy states. We will review which are the most likely theoretical and experimental sources for this discrepancy and which are the state of this puzzle at the moment.