  We briefly review flavour violation in the lepton sector: starting from
neutrino oscillations and their implications, we consider several charged
lepton flavour violating observables at high and low energies. We present new
physics models and discuss the r\^ole of the latter in disentangling them. In
particular, we show how the interplay of different observables allows to derive
important information on the underlying mechanism of lepton flavour violation.
As an example, we discuss the impact of a type-I SUSY seesaw concerning lepton
flavour violation at low energies and at colliders (LHC and a future Linear
Collider).
