The present document deals with scaling in nuclear-system thermal hydraulics (SYS TH), including the connection with nuclear-reactor safety technology (NST). Scaling has constituted “an issue” since the beginning of the exploitation of nuclear energy for civil purposes, with main reference to the generation of electricity. A nuclear power plant (NPP) constitutes a technologically complex industrial system. There are several origins of its complexity, connected with the need to reduce the cost of producing electricity and to manage the radioactive fission products. This resulted, among the other things in the large pressure vessel, high power and, mainly, high power density (power per unit-core volume), high pressure, and the need for engineered safety-features, including an emergency core-cooling system. Then, another problem was the impossibility of, or the large difficulty in, characterising the system’s performance under the conditions of the design: almost unavoidably, to reduce the cost, the experiments aimed at understanding the original system, here called the prototype, were performed in small-scale systems herein called models. So, models were designed, constructed, and operated under downscaled ranges of values for one or more of the listed parameters. These features lay at the origin of the scaling issue, i.e. the difficulty in demonstrating that a model behaves like the prototype.