There is only one way to avoid traffic accidents and that is care exercised by everyone, particularly the drivers. Similarly reactor safety will depend on the people who operate the reactors. At the same time a lot of help can be obtained by careful construction and scrutiny of each new reactor. One of the first acts of the Atomic Energy Commission was to establish a Committee for Reactor Safeguards. With the passing of years this committee had to take on more heavy responsibilities. At first it had to operate under secrecy. With the wider and more public use of reactors the safety considerations are becoming more available to the public. The question of safe operation of a machine cannot be separated from a thorough understanding of the working of the machine. We cannot attempt to give an adequate description of a reactor or of the safety rules. A few general statements have to suffice. A working reactor is full of neutrons. In a small fraction of a second these neutrons produce fission and a new generation of neutrons comes into being. In slow reactors which contain lots of light elements like hydrogen or carbon, the neutrons move with speeds little greater than that of sound and a generation may last as long as a millisecond (one thousandth of a second). In fast reactors which contain almost exclusively heavier elements like uranium or iron, neutrons move with a great speed which is about three per cent of the speed of light. In this case one generation replaces another in less than a microsecond (one millionth of a second). Fortunately not all the neutrons get reproduced so rapidly. Some fissions produce delayed neutrons which are emitted usually with a delay of several seconds. In a steadily working reactor each generation should have the same number of neutrons as the previous one. If each succeeding generation has even a slight surplus, the reactor will become hot and may explode in a small fraction of a second. The main reason why safe operation is possible is the fact that fast multiplication can occur only if each generation becomes more populous _even when one does not count the delayed neutrons_. A slightly overactive reactor is easily governed, but there comes a point when the dormant dragon begins to stir. This happens when there are enough neutrons produced so that multiplication can occur without waiting for the delayed neutrons. At that point a well behaved dragon will perform a harmless action. For instance it may blow a fuse. But a vicious dragon will spit radioactive fire.