Except the radioactive sources, the positron sources are all based on photon materialization into electron-positron pairs in a target. After a rapid recall of the different methods to generate photons, underlining the corresponding photon characteristics , we shall concentrate on three kinds of radiation: bremsstrahlung in amorphous targets , channeling and coherent bremsstrahlung in oriented crystals. Applications to future linear e+e- and circular μ+ μ – colliders will be presented.