The photon reveals a structure stemming from quantum fluctuations. This structure is the subject of intensive experimental investigations. The present experimental knowledge is presented with emphasis on measurements from deep inelastic electron-photon scattering at electron-positron colliders. The presentation covers the
leptonic and hadronic structure of quasi-real and also of highly virtual photons, based on measurements of structure functions and differential cross-sections. The most important results from photon-photon scattering at LEP and from photon-proton and electron-proton scattering at HERA are also summarised.