The energy consumption in the world is directly related to the economical growth. The International Energy Agency estimates a doubling increase of electricity demands in the next 25 years ("Energy Outlook", 2004). Somehow, this growth must be satisfied without destroying the ecological equilibrium of our planet. For this reason, all sorts of energy resources, which are potential pollutants for the environment are being investigated, and their technologies improved, plus new renewable energies are developed. It is also evident that renewables will take several decades before they are able to substitute the present capacity and even more, to cope with the future energy demands. Nuclear energy is an air pollution free technology with the potential to satisfy the world's energy demands for many centuries. However, some concerns about the use of nuclear energy have to be further developed in order to recognize nuclear energy as a sustainable option. This book deals with concerns such as the use of nuclear energy for weapons, the risk of accidents with radioactivity release and the waste management.