This introductory chapter contains General remarks. The extremely high heat of solution of helium and the other inert gases in metals prevents the making of any metal-helium “alloy” by conventional methods. Investigations of helium atoms as defects in metals were therefore neither interesting nor feasible. This situation changed with the introduction of nuclear technology and ion beam methods where helium is produced by nuclear reactions between particles and metal nuclei and/or by direct implantation. Research in this area started in the mid-sixties, stimulated by a paper by Barnes who attributed observations of high temperature embrittlement of irradiated alloys to helium bubbles on the grain boundaries. About a decade later the field received a strong impetus when the progress in controlled thermonuclear fusion led to intense fusion materials research. It was soon recognized that the high energy fusion neutrons cause (n,α)-production rates which are more than one order of magnitude higher than those in fast fission reactors.