The selection, in 1954, of uranium dioxide as the fuel for the first commercial electric power-producing nuclear reactor started an intense research program which ultimately involved many people in many countries. It established fabrication procedures for this material and determined its behavior under use conditions in a reactor. Difficulties were encountered at first with making a high-density product and with a swelling of the product at high burnup. Later, an improved ability to make sinterable powder led indirectly to serious fuel shrinkage in use in some reactors. The problems were quite rapidly overcome and uranium dioxide is now the fuel for reactors generating over 2000 gigawatts of electrical power worldwide.