[en] More and more nuclear power reactors are coming into operation in different parts of the world. In June 1962, 19 power stations utilizing five reactor types with a total capacity of 1600 MW were in operation in six countries. By the end of this year there are expected to be 55 stations in ten countries with a total capacity of 4500 MW. In another three years the total capacity is likely to go up to 9500 MW, and at the end of the present decade to 15 000 - 18 000 MW. Many developing countries are interested in determining the most advantageous time for starting power reactor programmes. The data required for such a decision cannot be summarized in a single document, but some guidance in the analysis and interpretation of such data can be obtained from a consideration of the technical status of different power reactors, their cost aspects, the materials required for nuclear power production, and the steps to be taken by a country interested in a nuclear power programme. The power reactors to be considered may be divided into three broad categories: (a) those already in commercial use, (b) those considered promising for the near future, and (c) those based on advanced concepts, whose suitability for industrial use still needs extensive technological and experimental development