Thermionic emission is the thermally induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the work function of the material. The number of freely moving charge carriers in the metal, also known as electrons, that have sufficient energy to leave its surface is well described by the Dushman-Richardson equation. The history and the discoveries made by scientist investigating the photoelectric effect are closely related to the history of thermionic emission. The only discrepancy is the method via which the sufficient additional energy is provided. The key contributions were made by Einstein and his interpretation of the photoelectric phenomenon and Sommerfeld and his application of Fermi-Dirac statistics to the problem of moving electrons inside the metal. The ways in which we benefit from the discovery of the thermionic effect are many. Today they are used in special high-precision detectors for atomic laser spectroscopy. They also hold much promise as the future power suppliers of additional energy in spacecrafts through the technology of thermionic energy converters being developed today. Lastly, we have the discovery of the effect to thank for the electronic revolution and the world as we now know it. Had it not been for the discovery of electronic tubes used in pioneering radio broadcasting, the need for developing transistors, pillars of our technology today, may have never been realized.