This unique volume gives an accurate and very detailed description of the functioning and operation of basic nuclear reactors, as emerging from yet unpublished papers by Nobel Laureate Enrico Fermi. In the first part, the entire course of lectures on Neutron Physics delivered by Fermi at Los Alamos is reported, according to the version made by Anthony P French. Here, the fundamental physical phenomena are described very clearly and comprehensively, giving the appropriate physics grounds for the functioning of nuclear piles. In the second part, all the patents issued by Fermi (and coworkers) on the functioning, construction and operation of several different kinds of nuclear reactors are reported. Here, the main engineering problems are encountered and solved by employing simple and practical methods, which are described in detail. This seminal work mainly caters to students, teachers and researchers working in nuclear physics and engineering, but it is of invaluable interest to historians of physics too, since the material presented here is entirely novel.