Fusion, which powers the sun and stars, is potentially an environmentally responsible and intrinsically safe source of essentially limitless energy on earth. The potential of fusion has been recognized for over 65 years, but mastering fusion on earth has proved to be an enormous scientific and technical challenge. It involves heating a large volume of dilute gas, containing equal parts of deuterium and tritium, to over 100M°C (M°C=one million degrees celsius) while preventing it from being cooled by touching the walls, from which it must be isolated using a ‘magnetic bottle’. This has now been done, and the Joint European Torus (JET) – which is the world's leading fusion research facility – has produced 16 MW of fusion power. The next step, which is to build a power station sized device called ITER, will be taken by a global collaboration. ITER will be twice as big as JET in linear dimensions, and will integrate all the technologies needed in a fusion power station. ITER should produce at least 500 MW of fusion power, ten times more than needed to heat the gas, and confirm that it is possible to build a fusion power station. Time is, however, needed to further develop the technology in order to ensure that it would be reliable and economical, and to test in power station conditions the materials that will be used in its construction, which will have to stand up to intense bombardment by the neutrons that carry the energy out of the magnetic bottle. Up to now, fusion has not been developed with any sense of urgency: since devices called tokamaks emerged in 1969 as the best candidates for bottling hot gases, at least 15 years have been lost due to delays in decision making and inadequate funding. In view of the urgent need for new, large-scale, emission-free sources of energy, and given the fact that – assuming it can be made to work reliably – the economics of fusion power look reasonable, the time has come to develop fusion on the so-called Fast Track. This involves: building ITER and the essential International Fusion Materials Irradiation Facility in parallel, which will take ten years; using the results to finalize the design of a prototype fusion power station (generally called DEMO for Demonstrator); and then constructing DEMO, which will take another ten years. Assuming adequate funding, and that there are no major surprises, DEMO could be putting electricity into the grid within 30 years.