The development of nuclear energy both for weapons and more particularly for peaceful applications is essentially a new branch of applied science. Like many new engineering projects some of the limitations to future progress depend on metallurgical difficulties. Problems arising in the construction of the large chemical plants for any nuclear power project are essentially the same in kind, although perhaps rather more acute in character, as those in normal chemical industry where dangerous and toxic chemicals are involved. On the other hand, the manufacture of fuels for nuclear reactors has required a new industry to be set up for the extraction of uranium, and other new metals are becoming increasingly important if progress is not to be retarded. Apart from new metals, new alloys and higher quality material in conventional metals have been required for the protection of uranium from attack by the coolants used. From the earliest days this has been one of the acute problems of nuclear energy. Finally the metallurgical problem peculiar to atomic energy is the development of materials which will withstand the effects of neutron irradiation. The effects on non-fissile materials are usually minor changes in physical properties, but on fissile material the effects are much more severe. At lower temperatures the anisotropic nature of uranium results in large dimensional changes which are being overcome by grain refinement techniques involving alloying and heat treatment. At high temperatures the gaseous fission products tend to expand and disrupt the material by mechanisms which are not yet fully understood, and for which cures are still being developed.