The Proton Radius Puzzle- Why We All Should Care*

Electron and muon probes have separately been used to measure the radius of the proton, but different results have been obtained. This difference has been denoted the proton radius puzzle. The main interest in this subject is the intriguing possibility that violations of electron-muon universality exist. Recent experiments and the various possible solutions, which involve experimental, atomic, nuclear and particle physics, will be reviewed.