What is lacking in order to design and build a commercially viable fusion reactor?

A critical overview of the state of the art in fusion research is presented. The four main areas of activity are considered: physics, technology, engineering and safety. In physics, the trend is positive towards a better understanding of the suitable plasma regimes, which have to be confirmed through further experimentation on the operating machines. The quality of the engineering has already been demonstrated through the design and construction of a number of experimental machines, and a large database obtained from the design and operation of fission reactors is available. For safety very satisfactory results in analysing the impact of fusion on man and the environment are being achieved. In technology, an outstanding problem is that of finding low activation materials capable of resisting the harsh fusion environment for an adequate number of years. An intense 14 MEV neutron source is needed in order to allow the necessary developments.