  Positron sources are the key elements for the future and current lepton
collider projects such as ILC, CLIC, SuperKEKB, FCC-ee, Muon Collider/LEMMA,
etc., introducing challenging critical requirements for high intensity and low
emittance beams in order to achieve high luminosity. In fact, due to their
large production emittance and constraints given by the target thermal load,
the main collider parameters such as the peak and average current, the
emittances, the damping time, the repetition frequency and consequently the
luminosity are determined by the positron beam characteristics. In this paper,
the conventional positron sources and their main properties are explored for
giving an indication to the challenges that apply during the design of the
advanced accelerator concepts. The photon-driven positron sources as the novel
approach proposed, primarily for the future linear colliders, are described
highlighting their variety and problematic.
