This review presents an overview of electron ionization time-of-flight mass speclroscopy (EITOFMS), beginning with its early development to the employment of modem high-resolution electron ionization sources. The EITOFMS is demonstrated to be ideally suited for analytical and basic chemical physics studies. Studies of the formation of positive ions by electron ionization time-of-fiight mass speclroscopy have been responsible for many of the known ionization potentials of molecules and radicals, as well as accepted bond dissociation energies for ions and neutral molecules. Vie application of TOFMS has been particularly important in the area of negative ion physics and chemistry. A wide variety of negative ion properties have been discovered and studied by using these methods including: autodetachment lifetimes, metastab/e dissociation, Rydberg electron transfer reactions and field detachment, SFa Scavenger method for detecting temporary negative ion states, and many others.