The coming decades will bring big changes in energy systems throughout the world. The systems are expected to change from central power plants producing electricity and maybe heat for the customers to a combination of central units and a variety ofdistributed units such as renewable energy technologies or fuel cells. Furthermore the following developments are expected: -closer link between supply and end-use -closer link between the various energy carriers distributed through grids such aselectricity, heat, natural gas and maybe hydrogen in the future -increased energy trade across national borders Risø Energy Report 4 is the fourth in a series of reports covering energy issues at global, regional and national levels. This report coversthe future of energy systems over the next 20–30 years. It deals with sustainable energy in general, but pays special attention to system aspects and the distribution of energy through grids such as those used for natural gas, electricity, districtheating and hydrogen. The focus is on industrialised countries, but the report also deals with specific points relevant to developing countries, such as isolated energy systems. The transport sector is discussed only in the context of its use of energysupplied through the various grids. Individual chapters of the report have been written by Risø staff members and leading Danish and international experts. The report is based on internationally recognised scientific material, and is fully referenced andrefereed by an international panel of independent experts. Information on current developments is taken from the most up-to-date and authoritative sources available. Our target groups are colleagues, collaborating partners, customers, fundingorganisations, the Danish government and international organisations including the European Union, the International Energy Agency and the United Nations.