The Monte Carlo method is a numerical approach to solve physical problems. It is based on statistical techniques and it is very useful to describe complex systems that cannot be fully modelled in a realistic way using deterministic methods. Therefore, by using Monte Carlo techniques it is possible to describe a complex process by means of its individual events, in such a way that the target physiscal magnitude can be described in the large-number-of-events limit. This imposes a computational limitation that requires the use of sofisticated programing languajes and state-of-the-art computing facilities. Three different Monte Carlo codes are used in the group: Monte Carlo code for Neutron and Photon transport MCNP, PENetration and Energy LOss of Positrons and Electrons PENELOPE, and GEometry ANd Tracking GEANT4.