In the recent years there is an increasing interest towards the positron annihilation spectroscopy as an examination technique in many fields of knowledge. Its basic applications are for scientific investigation in physics and materials science, because the method is non-destructive and very sensitive to different kinds of defects and phase transitions, but it found its place also in some practically aimed activities like medicine. In the present book are represented investigations of metals and alloys irradiated with 14 MeV generator neutrons. The investigated metals are of the most often used ones for the purposes of the nuclear reactor industry. The discussion goes mainly around the clusters formation in them as a result of neutron irradiation and their growing and disappearance in high temperature annealing. The methods of the positron spectroscopy, experimental setup used in the measurements and mathematical methods for data analysis are also mentioned. The obtained results may be very useful not only for scientists, but for all specialists in the field of nuclear energetics - projectors, constructors and nuclear engineers as well as for mechanical engineers and constructors.