In the second half of the 20th century, nuclear power evolved from the research and development environment to an industry that supplies 16% of the world's electricity. Global environmental change and the continuing increase in global energy supply required to provide increasing populations with an improving standard of living, make the contribution from nuclear energy even more important for the next century.  For nuclear power to achieve its full potential and make its needed contribution, it must be safe, economical, reliable and sustainable.  To achieve this, a wide spectrum of scientific and engineering personnel is required, with a broad understanding of the technology, as well as in-depth understanding of their respective specialities. To contribute to the training of scientific and engineering personnel, the IAEA sponsors the development of educational simulators that operate on personal computers and which simulate responses of a number of water-cooled reactor types to operating and accident conditions. The simulators are designed to provide insight and understanding of the general design and operational characteristics of various power reactor systems. The purpose is to provide university professors and engineers involved in teaching topics in nuclear energy with tools to demonstrate reactor operational response characteristics. The tools are also supplied directly to students, junior engineers, and senior engineers and scientists interested in broadening their understanding of the topic. (Note that the simulators are not intended for plant-specific purposes such as design, safety analysis, licensing or operator training, and they are not designed to link with control system components).