Massive neutrinos and their mixings provide the first evidence of flavour violation in the leptonic sector and open a whole window on many new processes, within reach of high-energy and high-intensity facilities. An observation of charged lepton flavour violation (cLFV) processes will undoubtedly point towards the existence of New Physics. Analogously, if observed, lepton number violating (LNV) processes are a clear sign of New Physics which indirectly hint for a Majorana nature of neutrinos. In this talk I will firstly briefly review the experimental status of cLFV searches. Then I will discuss the potential of minimal extensions of the Standard Model (SM) via heavy sterile fermions concerning an extensive array of observables. Among others, I will discuss cLFV processes like three-body decays, nuclear-assisted processes and rare LFV Z decays and LNV processes like “neutrinoless” meson and tau decays.