Proton decay: Spectroscopic probe beyond the proton drip line

Proton decay has been transformed in recent years from an exotic phenomenon into a powerful spectroscopic tool. The frontiers of experimental and theoretical proton-decay studies will be reviewed. Different aspects of proton decay will be illustrated with recent results on the deformed proton emitter Tb, the odd-odd deformed proton emitter Eu, the complex fine structure in the odd-odd Tm nucleus and on excited states in the transitional proton emitter Tm.