If measures to reduce greenhouse gas-emissions are taken and if fission will be phased out at the beginning of the 22nd century fusion has a good chance to win considerable shares of the electricity market. A detailed model of the Western-European energy system was made using the MARKAL toolkit. Scenarios were designed to understand the conditions that would make fusion a viable electricity production technique. Globally it is very likely that at least the first half of the 21st century will still be dominated by the use of fossil fuels, with a shift away from oil to natural gas. Energy and electricity demand are likely to increase over the complete 21st century.