Theoretical background for bileptonic gauge bosons is reviewed - both the S
U(15) GUT model and the 3-3-1 model. Mass limits on bileptons are discussed
coming from e(+)e(-) scattering, polarized muon decay and muonium-antimuon
ium conversion. Discovery in e(-)e(-) at a linear collider at low energy (1
00 GeV) and high luminosity (10(33)/cm(2)/s) is emphasized.