The results are reported of neutron emission measurements during fatigue experiments at low (2 Hz), intermediate (200 Hz), and high (20 kHz) frequencies. These results confirm that appreciable neutron emissions, without any doubt greater than the natural background level, are observed during damage accumulation in iron-bearing rocks: granite (Fe ~1.5%), basalt (Fe ~15%), and magnetite (Fe ~75%). The neutron detection, together with temperature measurements obtained by infrared techniques, leads to the conclusion that fatigue tests performed at 200 Hz represent the condition for which the neutron emission is the highest. This evidence is corroborated by the seismological observations at Jacinto fault in Southern California, as well as by the neutron emissions detected by the author during seismic activity. On the other hand, as will be explained later in the book, it is only a component of the frequency spectrum that is directly triggering the phono-fission reactions, i.e., the TeraHertz range component. This component is represented by phonons produced by micro and nanocracking and not directly by an instrument or a seismic vibration.