This book is something which almost accidentally has developed very differently from how it was initially planned. The intention was to elaborate the part played by the immune system in ageing with the role of the thymus as central theme. It was to be essentially an expansion of a lecture I gave in 1970 and would inevitably have been concerned with much the same material as Walford's book, The Immunologic Theory of Aging, though from a different slant. What changed its character arose from a series of attempts to find logical connection between two findings that most gerontologists regard as that the lifespan of a mammal is genetically determined, and that the actual process of ageing is an accumulation of genetic error, of somatic mutations. It is possible that the connection is so indirect, circuitous and multiform that generations of detailed and unattractive research will be needed to elucidate it, or, more likely, the whole matter discarded as a non-problem. But a more inspiring approach does seem possible. The working hypothesis, which halfway through its writing became the new central theme of the book, arose when I was a member of a committee appointed by the Australian Academy of Science at the request of the Australian Government to advise on the danger from French nuclear tests in the South Pacific.