Book excerpt: Multi-Physics Modelling (MPM) is an innovative simulation technique that looks very promising for the employment in the field of nuclear engineering as an integrative analysis support in the design development of current and innovative nuclear reactors. This book presents a Multi-Physics Modelling (MPM) approach to the analysis of nuclear reactor core behaviour, developed to study the coupling between neutronics and thermo-hydrodynamics. Reference is made to the Molten Salt Reactor, one of the innovative nuclear systems under development in the framework of the Generation IV International Forum, but the same methodology can be applied to other reactor systems.