A history of developments in the uses of radioactivity and ionising radiation in the last century and the measures used to protect people from harmful effects. The sources of radiation exposure covered include medical uses, nuclear power generation and natural ones, such as that from radon. The book traces the evolution of our understanding of the effects of radiation on the human body, particularly those leading to cancer and hereditable diseases. One of the key challenges for practitioners was constructing manageable frameworks for evaluating these effects so that there could be effective control and regulation. The book emphasises the international nature of these efforts and the need to change approaches as more data became available after the Second World War. For the second edition the opportunity has been taken to revise the citation system and make some correction but the major change is the addition of two new Chapters: one on nuclear criticality safety and the other on nuclear safety assessment.