  This is a review of the cosmological bounds on $B-L$ violating interactions,
and the loopholes in the argument that gives these constraints. If one assumes
that the baryon asymmetry we observe today was present above the electroweak
phase transition in equilibrium with the non-perturbative $B+L$ violating
processes, then interactions that violate all three of ${B/3 - L_i}$ cannot
simultaneously be in equilibrium. Otherwise the baryon asymmetry would be
washed out. Therefore violation of at least one of the $B/3 - L_i$ must be
small. This argument can be evaded by not having the observed baryon asymmetry
present in the thermal bath (for instance, make it at the electroweak phase
transition), by using a non-standard cosmological model, or possibly by some
mass effects in models where the difference between lepton flavour asymmetries
is conserved.
