Overdiagnosis is an emerging problem in health policy and practice: we address its definition and ethical implications. We argue that the definition of overdiagnosis should be expressed at the level of populations. Consider a condition prevalent in a population, customarily labelled with diagnosis A. We propose that overdiagnosis is occurring in respect of that condition in that population when the condition is being identified and labelled with diagnosis A in that population ; this identification and labelling would be accepted as (...) correct in a relevant professional community; but the resulting label and/or intervention carries an unfavourable balance between benefits and harms. We identify challenges in determining and weighting relevant harms, then propose three central ethical considerations in overdiagnosis: the extent of harm done, whether harm is avoidable and whether the primary goal of the actor/s concerned is to benefit themselves or the patient, citizen or society. This distinguishes predatory, misdirected and tragic overdiagnosis; the degree of harm moderates the justifiability of each type. We end with four normative challenges: methods for adjudicating between professional standards and identifying relevant harms and benefits should be procedurally just; individuals, organisations and states are differently responsible for addressing overdiagnosis; overdiagnosis is a matter for distributive justice: the burdens of both overdiagnosis and its prevention could fall on the least-well-off; and communicating about overdiagnosis risks harming those unaware that they may have been overdiagnosed. These challenges will need to be addressed as the field develops. (shrink)