The analysis of neutron and photon propagation is the fundamental tool for the study and design of nuclear fission and fusion reactors. Although neutronics is based on a well-known linear equation, its solution for problems of engineering interest requires the development of dedicated techniques. This course aims to discuss and analyze the computational codes and methods currently used for the design and safety analysis of nuclear reactors. The case studies considered are pressurized water light-water fission reactors and large magnetic confinement fusion machines under construction, starting from the identification of the observable quantities needed for the design up to the corresponding calculation chain needed to determine them. The peculiarities of deterministic and probabilistic calculation methods are discussed, highlighting the complementarity of their use. For fission reactors, the calculation schemes based on the two-step method for stationary problems are analyzed. The different techniques for solving the transport equation for lattices of fuel elements and for solving the diffusion equation for complete reactors are described. For fusion reactors, the problems of shielding are highlighted. The course is based on the direct application of the numerical techniques presented in simplified problems. Lectures will be accompanied by exercises to be carried out independently on case studies. At the end of the course, a project is discussed concerning the programming of a solver and/or the use of an available open-source code. A basic competence in computer programming, for example in Python, is recommended.