This thesis is about screening for cancer—about testing healthy individuals for disease. The traditional way to approach this subject is to begin by considering whether screening is effective. So, for example, there is a hearty debate about whether cancer screening reduces mortality to a meaningful degree. Some people claim that it does; others claim that it does not. But all seem to think that determining whether screening reduces mortality will resolve the controversy around screening. The motivating idea behind this thesis is that focusing on effectiveness obscures a deeper and different set of issues. The central claim is that understanding and justifying cancer screening requires attention to its moral dimensions. Ethical issues arise throughout the debate— not only in constructing a theory of effectiveness, but also in deciding evidentiary thresholds, in adjudicating which principles should guide screening policy, and in judging how to cope with risk and uncertainty. The subsequent chapters aim to show how significant progress and clarity can be achieved in the screening debate with some aid from ethics. Taken together, these chapters develop a framework for thinking about how cancer screening can and should be justified.