Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In nuclear physics, nuclear fusion is a nuclear reaction in which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. During this process, matter is not conserved because some of the mass of the fusing nuclei is converted to energy which is released. Research into developing controlled thermonuclear fusion for civil purposes also began in earnest in the 1950s, and it continues to this day. Learn more about the principles of nuclear fusion and its potential as an energy source in this book.