The Integral Fast Reactor (IFR) is a fast reactor system developed at Argonne National Laboratory in the decade 1984 to 1994. The IFR project developed the technology for a complete system; the reactor, the entire fuel cycle and the waste management technologies were all included in the development program. The reactor concept had important features and characteristics that were completely new and fuel cycle and waste management technologies that were entirely new developments. The reactor is a "fast" reactor - that is, the chain reaction is maintained by "fast" neutrons with high energy - which produces its own fuel. The IFR reactor and associated fuel cycle is a closed system. Electrical power is generated, new fissile fuel is produced to replace the fuel burned, its used fuel is processed for recycling by pyroprocessing - a new development - and waste is put in final form for disposal. All this is done on one self-sufficient site. The scale and duration of the project and its funding made it the largest nuclear energy R and D program of its day. Its purpose was the development of a long term massive new energy source, capable of meeting the nation's electrical energy needs in any amount, and for as long as it is needed, forever, if necessary. Safety, non-proliferation and waste toxicity properties were improved as well, these three the characteristics most commonly cited in opposition to nuclear power. Development proceeded from success to success. Most of the development had been done when the program was abruptly cancelled by the newly elected Clinton Administration. In his 1994 State of the Union address the president stated that "unnecessary programs in advanced reactor development will be terminated." The IFR was that program. This book gives the real story of the IFR, written by the two nuclear scientists who were most deeply involved in its conception, the development of its R and D program, and its management. Between the scientific and engineering papers and reports, and books on the IFR, and the non-technical and often impassioned dialogue that continues to this day on fast reactor technology, we felt there is room for a volume that, while accurate technically, is written in a manner accessible to the non-specialist and even to the non-technical reader who simply wants to know what this technology is.