Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, and is represented by the symbol He. It is a colourless, odourless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions. Next to hydrogen, it is the second most abundant element in the universe, and accounts for 24% of the elemental mass of our galaxy. This book presents current research data in the study of helium, including helium in the terrestrial upper atmosphere; an experimental and analytical investigation of helium in metals diffusion; the release of mantle helium and its tectonic implications; and the study of helium behavior in nuclear fuel materials.