Most of the attention in this book has been devoted to neutrons. It is neutrons that determine the fission rate and core criticality, so this emphasis is entirely proper. However, when shielding or decay heat problems are to be solved then another neutral entity, the gamma rays, have to be considered. An operating (and also a shutdown) nuclear reactor core produces large numbers of gamma rays. They can be divided into three general categories: the ‘prompt’, the ‘delayed’ and the ‘activation’ gamma rays.