What we have to discuss in this book is connected with parts of atomic and nuclear physics which are much more elementary. The facts which we shall present in a simple fashion are sufficient to give the reader an orientation in the seemingly bewildering fields of nuclear energy and atomic explosions. We shall have to start by describing atoms and nuclei. These are rather small objects, but this circumstance need not particularly bother us; and it is not necessary to frighten ourselves with the idea that we are talking about “unimaginably” small objects. Our minds adapt themselves quite readily to new dimensions; and while we are talking about nuclei, we can temporarily forget that any bigger objects exist. Real difficulties arise only when science discovers laws which seem to contradict common sense. This does not happen frequently, and we shall not need to dwell upon such subjects. The difficulties of explaining science are increased by the fact that scientists have developed a language of their own which they practice and perfect by talking to each other. One sometimes has the impression that they talk to each other exclusively. The authors feel that their own “native tongue” is this scientific language; this book is an effort at a translation. A further difficulty is connected with the special subject: radioactivity. The great practical importance of this subject has dawned upon the public in connection with the explosion at Hiroshima. This was a frightening occasion, and the subsequent developments and prospects are no less frightening. It is not necessary that everything connected with nuclear explosions should be equally frightening; and it is important that we should approach the subject with an open mind and with as few emotions as is humanly possible. The emotions have their necessary place when we get to the stage in which we want to decide our actions. We suggest to the reader that he should delay this stage until the time when he has finished reading the book. The greatest difficulty in discussing the radiation hazards arises because the working of living organisms is involved. Basically, we are in the dark about the question how such an organism works. We are equally in the dark about the question how such an organism is affected by radiation. It would therefore seem that we must remain in doubt whether or not radioactivity is dangerous, except for those cases where obvious damage has been done. Since the immediate effects of radioactivity are not perceived by our senses, we are faced with the thought of an invisible menace of unknown extent. Some of the harmful consequences may show up years later, and therefore even the absence of any observed damage will not reassure people.