There is no doubt that nanoscience will be the dominant direction for technology in this century, and that this science will influence our lives to a large extent as well as open completely new perspectives on all scientific and technological disciplines. To be able to produce optimal nanosystems with tailor-made properties, it is necessary to analyze and construct such systems in advance by adequate theoretical and computational methods. Since we work in nanoscience and nanotechnology at the ultimate level, we have to apply the basic laws of physics. What methods and tools are relevant here? The book gives an answer to this question. The background of the theoretical methods and tools is critically discussed, and also the “world view” on which these physical laws are based.Such a debate is not only of academic interest but is of highly general concern, and this is because we constantly move in nanoscience and nanotechnology between two extreme poles, between “infinite life” and “total destruction”. On the one hand, through nanotechnology aging might be soon a fact of the past; on the other hand, in the nano realm uncontrolled processes could lead to a total destruction of the living conditions on the Earth.