  Bosons which couple to two leptons and carry lepton number L=2, generically
called bileptons, appear in many extensions of the standard model. We review
the present constraints on bilepton parameters coming from low and high energy
data. In particular, we point out that the amplitude of $\mu-e$ conversion in
nuclei is enhanced by large logarithms if compared with the $\mu\to e\gamma$
one. Bilepton couplings will be tested most stringently at future colliders.
For wide range of neutrino masses the cross section of one of the processes
$e^-e^- (\mu^-\mu^-)\to \ell_i^-\ell_i^-,$ $i=e,\mu, \tau,$ mediated by scalar
triplet bilepton, has such a lower bound that it should be observed at future
facilities.
