The Prototype Fast Reactor (PFR) at Dounreay was the centrepiece of the UK fast reactor development project, the largest single government-funded civil research & development activity of the post-war years, when the UK nuclear industry was among the world leaders. Now decommissioned, PFR played a major part in demonstrating that sodium-cooled fast reactors can play an essential part in the nuclear power industry of the future.Tony Judd tells the story of the PFR's design and operation, its faults and achievements. His account is based partly on published documents (official reports, papers, and conference proceedings), partly on unpublished documents in the UK National Archive, and partly on the memories of the people who designed, built, and wrestled to understand this complex machine. The UK Prototype Fast Reactor is the insiders' story, told by engineers and scientists rather than directors and politicians. It is about the successes and challenges and, importantly, it shows that PFR was far from being a failure. It is frank about the difficulties that were overcome in making it a hard-won success as a power station and a test facility. It draws important lessons for the future of an industry which is essential to our energy security.