This book appears a century after the discovery of radioactivity. It was in 1896, when Henri Becquerel reported his first results about the penetrating radiation, which could darken the packed photographic plates. The initial fascination of radioactivity, e.g., the discovery of new radioactive elements, the first real description of the structure of atoms and their nuclei, the applications of radiotracers, the high sensitivity of activation analysis, etc., was followed by the use of atomic bomb in 1945. The mushroom cloud became a symbol of destructive nuclear power. And even nuclear energy production (which provides about 20% of the world's electricity) is overshadowed by radioactive waste. However, the latest results suggest that the Accelerator-Driven Transmutation Technology (ADTT) will solve this problem, since this technique can decrease the lifetime of the fission products comparatively to the human lifespan. Practical control of fusion may also be possible in the first decades of the next millennium.