  Theoretical backgound for bileptonic gauge bosons is reviewed, both the
SU(15) GUT model and the 3-3-1 model. Then the mass limits for bileptons are
discussed coming from $e^+e^-$ scattering, polarized muon decay and
muonium-antimuonium conversion. A consequence of precision electroweak data on
the masses is mentioned. Discovery in $e^-e^-$ at a linear collider is
emphasised.
