Concerning small and medium sized reactors, the countries concerned have been increasingly interested in their safety, public acceptance and economy etc. In this review, the development and its strategy of the concerned countries on these reactors are described depending upon the papers of the First International Seminar on Small and Medium-Sized Reactors held last summer in Laussanne, informations obtained with discussions including industrial people who are designing small and medium sized reactors and already published papers. Reactor safety is classified into four safety items for pointing out the reactor safety characteristics.