The Physics of Same-Sign Lepton Colliders ========================================= Clemens Heusch (Univ.Calif.Santa Cruz) Abstract: --------- For the next generation of particle accelerators beyond the LHC, electron and muon colliders are being studied for their technical feasibilities at high energy and luminosity. We review the physics promise as well as some critical issues of their realizability, for the case where both incoming beams have the same electric charge. It turns out that electron and muon colliders offer very different chances and problems.