Inverse neutrinoless double beta decay (and other Delta L=2 processes)

I review the prospects for the detection of Delta L=2 processes at future colliders. Except in contrived models, the process e- e- -> W- W- is unobservable at future linear colliders unless $\sqrt{s} \gsim 2$ TeV, due to constraints from neutrinoless double beta decay. As there are no analogous constraints on the Majorana mass of the $\nu_\mu$, mu- mu- -> W- W- could be observed at a muon collider with considerably lower $\sqrt{s}$. One can also consider esoteric processes such as gamma gamma -> mu+ mu+ W- W-. Such processes may be observable if $\sqrt{s} \gsim 4$ TeV.