The results of a systematic qualitative and quantitative study of the little understood phenomenon of fatigue in photomultiplier tubes are presented in this paper. A precision of 0.4% was obtained in the measurements. After having determined the variables that affect fatigue and the mathematical dependence of fatigue with each of them, the general properties of fatigue and the nature and mechanism of the effect are discussed. Photomultiplier fatigue appears as a field emission from the dynode bombardment, probably combined with diffusion or conductivity through the thin insulator film of the dynode surface.