The International Project on Innovative Nuclear Reactors and Fuel Cycles (INPRO) is an international project to help insure that nuclear energy is available to contribute in a sustainable manner to meeting the energy needs of the 21st century. This publication was prepared to ease the application of the INPRO methodology, an IAEA tool that assists Member States in strategic planning and decision making on nuclear power programmes. It provides an overview of the methodology and recommendations on how to perform a nuclear energy system assessment using it. Different users of the INPRO methodology are addressed: nuclear technology developers, experienced nuclear technology users and prospective first time users of nuclear power technology.