In exploring early detection, we consider the usual model to be one in which cancer develops after a series of invisible steps in a single cell or a series of cells in the same milieu, perhaps in a sensitive individual. The object of the experiment is to develop cells that can be used in further experiments. In life, however, it is probable that most cell changes – even those that would lead to neoplasia – in fact regress. Thus, we are presented with the conundrum that we want to detect disease early, but not too early. There may be a universality about these early detection markers, or they may be very specific. When considering how imaging can help with early detection, we need to think about risk stratification of the population. It is not possible to image the whole population routinely, no matter what the technique. The imaging technologies considered in this volume cover a wide range of the electromagnetic spectrum and are used in a wide variety of settings. Our hope is that producing the chapters has been a thought-provoking exercise for the authors and that reading the chapters will prove likewise for the readers.