The aim of this book, sent to press just before the Fukushima event, is to give an engineering panoramic view of the nuclear energy applications, concerning both fission and fusion reactors. This book represents a second edition, widely revised and extended, of a previous one, published in 2008 and rapidly become out of print. The book is composed by four chapters and at the end of each chapter, a selection of certain subjects, physical, historical, etc. are deepened. Several numerical applications are presented too. In Chapter 1 some elements of nuclear physics are recalled, in order to introduce the reader into the world of nuclear technologies. Some curious information on the uranium mines present in Italy are reported too. In Chapter 2...