[en] During the 1960s and the early 1970s. the nuclear power plants entering service were dominated by plants with outputs falling into the small (less than 150 W e ) and medium (300 MWe to 700 MWe) reactor size ranges. During the late 1970s and 1980s, the balance shifted to large size plants (900 MWe to 1400 MWe), as nuclear power plants entered service, predominately, to serve the requirements of industrialized countries such as the US, Japan, Germany, and France. However, in the 1990s, the pendulum has swung back in the direction of small and medium sized reactors; currently 65% of the nuclear power plants under construction fall into the small and medium reactor size ranges. This shift has resulted from a sharp reduction in the number of nuclear power plants being built in the industrialized countries, in combination with the continued construction of small and medium sized nuclear power plants in developing countries such as India. China, Pakistan, and Slovakia. Developing countries are often characterized by limited capacity electrical grid systems. limited financial capability, and rapidly expanding energy demand requirements. In addition, most of the developing countries in which rapid increases in population and energy demand are occurring have few or very limited indigenous energy resources. These countries are therefore very interested in acquiring energy sources (for electricity production and process heat) that can serve the needs of their people and industries, and which do not overburden their financial capabilities (balance of payments, national debt load. etc.). It is therefore not surprising that many developing countries have, particularly during the late 1990s, expressed an interest in small and medium sized nuclear power plants (SMRs). The requirements for SMRs often cited by developing countries include low absolute capital cost, short construction schedule, favourable economic operation, and infrastructure requirements within the technical and economic capability of their country. This paper reviews key issues that impact the technical and economic viability of Small and Medium Reactors (SMRs), provides information on trends world wide that are favourable to the application of SMRs, reviews technologies that are available to Small Reactors, and provides an overview of the potential applications of SMRs. A review of SMR related activities of the IAEA and member states is also provided. (author)