Transmutation of long-lived actinides and fission products from nuclear waste, plutonium from nuclear weapons, or thorium as an energy source are being investigated with the increasing interest in the last two decades. Various concepts of transmutation also involve Accelerator Driven Systems based on a subcritical nuclear reactor driven by an external spallation neutron source. This work is a part of a complex research within the framework of the international project "Energy plus Transmutation" investigating spallation reactions, neutron production and transport and their usage for transmutation purposes. The book provides comprehensive information about the subject of nuclear spallation, detailed description of activation method and gamma-spectroscopy, explanation of Monte Carlo simulations with illustrations. The book can be used as a textbook for students of nuclear physics and engineering, while the elaborative discussion and comparison with data of others measured to date can be of interest for researchers in nuclear spectroscopy and anyone interested in neutron physics and Monte Carlo simulations.