The building of more nuclear power plants is one of the proposed solutions to the increasing production of anthropogenic CO2 and the mitigation of the global warming threat. Had the USA constructed an additional 56 nuclear power plants in the 1990s, the country would have been in compliance with the Kyoto protocol. Nuclear power plants are typically very large (1,000 MW) and very complex power producing units. The nuclear reactor itself contains large amounts of radioactive materials, which if released in the environment, may cause large-scale environmental accidents. For this reason, a nuclear reactor must have multiple levels of safety systems and its controls must be designed to shut down the reactor within a very short time. All nuclear reactors have six basic components, which are described in detail in this chapter. Several types of nuclear reactors and nuclear power plants have been developed by different countries. The special design characteristics and the operation of these plants are summarized. Because accidents in nuclear power plants are feared the most by the public, the three accidents that received the highest notoriety, at the Three-Mile Island in the U.S.A., at Chernobyl in the former Soviet Union and at the Fukushima Dai-ichi power plant in Japan are described in detail. The causes of the accidents are examined and early actions that could have been taken by the operators are presented. Finally, it is apparent that, if the world is to rely on nuclear energy in the long-term, the more abundant uranium-238 and other fertile nuclear materials must be utilized. This makes necessary the use of breeder reactors, which may become the next generation of nuclear reactors.