These constraints led to the development of slow-electron velocity-map imaging (SEVI) in our laboratory. This technique was enabled by the many advances in ion and photoelectron imaging since the advent of ion imaging experiments in 1987, particularly the development of velocity-map imaging (VMI) by Eppink and Parker , first applied to negative ions by Sanov. In photoelectron VMI experiments, one typically images all of the photoelectrons onto a CCD camera and in principle can obtain the full photoelectron kinetic energy and angular distribution with a single measurement. In SEVI, we take advantage of the fact that the energy resolution of VMI is best for slow electrons, so our VMI electron optical setup is designed to preferentially detect electrons with low kinetic energies, yielding an energy resolution as high as 1-2 cm -1 for the slowest electrons for electrons with kinetic energy below 100 cm -1 . Further improvements result from trapping the anions in a radio frequency octupole trap held at 5 K and periodically filled with a low pressure He/H2 gas mixture. The trapped ions then undergo buffer gas collisions that cool them to internal temperatures as low as 10 K, resulting in nearly all the anions being in their vibrational ground state and low rotational states. This improved version of the experiment, cryo-SEVI, leads to near-complete suppression of vibrational hot bands and narrow rotational profiles, enabling one to obtain high resolution spectra even for complex molecular anions and clusters. Variants of anion SEVI and cryo-SEVI have been reported by several laboratories, including those of Gibson, Wang, Garand, Ning, and Heaven; the instruments used in those studies have also achieved sub-meV electron energy resolution. More specialized techniques such as photodetachment microscopy yield even better resolution but are less universally applicable.