Since photodetachment can access neutral electronic states of different spin multiplicity, PES provides a direct measure of singlet-triplet splittings and other spectroscopically challenging energetics. In addition to these appealing capabilities, the neutral species that one can access by photodetachment are of considerable interest. PES is well suited to study open-shell radicals, which can be difficult to study experimentally due to their transient nature, through photodetachment of their closed-shell anions. Since anion PES is inherently a mass-selective technique, it can also be applied to the anion clusters generated by laser-vaporization or electron impact sources and thus yield photoelectron spectra of size-selected clusters. Finally, photodetachment of stable anions can access the transition states of unimolecular and bimolecular chemical reactions, in which case anion PES serves as a powerful spectroscopic probe of transition states.