Overdiagnosis is a growing problem worldwide. Overdiagnosis is the diagnosis of deviations, abnormalities, risk factors, and pathologies that in themselves would never cause symptoms (this applies only to risk factors and pathology), would never lead to morbidity, and would never be the cause of death. Overdiagnosis is often misinterpreted as overutilization or overtreatment. Overutilization, overtreatment, and overdiagnosis are interrelated but three distinct topics. Overutilization (establishment of standard practice that does not provide net benefit) does not have to lead to overdiagnosis or overtreatment, but the risk exists. Treatment of overdiagnosed conditions is one category of overtreatment. Another is when the best available evidence shows that the treatment has no beneficial effect. Overdiagnosis can be caused by overutilization and is nearly always followed by overtreatment. Treating an overdiagnosed condition cannot improve the patient’s prognosis, and therefore can only be harmful. At the individual level, we can never be sure if the person is overdiagnosed. However, experiences and thoughts of individuals who are most likely overdiagnosed can be explored in qualitative interviews, e.g. men with a small screening detected abdominal aortic aneurism. In longitudinal surveys, the degree and length of psychosocial consequences associated with overdiagnosis can be estimated. In high-quality RCTs, the magnitude of overdiagnosis can be quantified, and in cohort studies, we can find indications of overdiagnosis. Finally, we can conduct research about the consequences of overdiagnosis in at least eight different areas: financial strain, hassles/inconveniences, medical costs, opportunity costs, physical harms, psychological harms, societal costs and work-related costs.