Objectives Overdiagnosis is a complex problem that is challenging to conceptualise and define. This challenge contributes to problems with measuring it and developing ways to decrease rates of overdiagnosis. The objective of this study is to identify the different levels at which overdiagnosis can be conceptualised and defined in order to improve information for patients’ decision making, estimates by health services researchers, and decisions of health policy makers.

Method Literature search (for definitions, conceptualisations, and estimates of overdiagnosis), qualitative content analysis, and philosophical concept analysis.

Results Overdiagnosis can be defined on three levels: personal, professional, and population. On the personal level overdiagnosis occurs when a person is given a diagnosis of a condition that otherwise would not have progressed to manifest disease. From a professional perspective overdiagnosis is linked to the definition of disease and can be identified in terms of dysfunction that is not at significant risk of causing harm. On a population level overdiagnosis is defined in terms of correct diagnoses that carry an unfavourable balance between benefits and harms. To make sense for individuals, overdiagnosis has to be explained (prognostically) in terms of the individual’s chance of being diagnosed (and treated) for what would otherwise have been manifest disease. Arriving at this information requires attention to professionals’ conception of disease, as well as epidemiologists’ estimates of the proportion of individuals who are diagnosed without overall benefits.

Conclusions On a personal level overdiagnosis involves being unnecessarily diagnosed with (and treated for) a disease. To better estimate the chance of this occurring for individuals we need agreement regarding professional conceptions of disease and population-based estimates.