Introduction In the long term, fusion power offers a secure, virtually resource-unlimited source of electricity supply. It has been found by independent review bodies to possess "inherent environmental and safety advantages over all current alternatives for base load electricity generation". The potential contribution of fusion power to long-term sustainable world development is, therefore, very great. However, it is not yet certain that fusion's potential can be realised in an economically acceptable form. Determining whether or not this can be done is the purpose of the world-wide fusion research programme. The programme aims to bring fusion technology to a commercially usable state for electricity generation by about the year 2040. Thus, fusion would be entering the market in a world in which the demand for electricity will be much greater than it is today, and in which reliance on fossil fuels is expected to be constrained by environmental imperatives. The case for pursuing fusion research is based on the major environmental and economic benefits which fusion power might supply.