Thermionic Phenomena is the third volume of the series entitled The Collected Works of Irving Langmuir. This volume compiles articles written during the 1920's and early 1930's, the period when the science of thermionics is beginning to be of importance. This text is divided into two parts. The first part discusses vacuum pumps, specifically examining the effect of space charge and residual gases on thermionic currents in high vacuum. This part also explains fundamental phenomena in electron tubes having tungsten cathodes and the use of high-power vacuum tubes. The second part of this text looks into the electron emission and adsorbed films, specifically studying the relation between contact potentials and electrochemical action and other related topics on electron emission. This publication will be invaluable to those interested in the works of Langmuir, particularly on thermionic phenomena.