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. Therefore, treating an overdiagnosed condition (deviation, abnormality, risk factor, pathology) cannot, by definition, improve the patient's prognosis, and can therefore only be harmful. Overdiagnosis is an extremely harmful and big problem all over the world, and the problem is increasing. This is especially the case in high-income countries, where more sensitive tests, more testing, more screening and earlier diagnosis is in focus, and more of the same will be implemented in the future. Moreover, disease definitions have been and are still being widened, plus thresholds for treating, e.g. risk factors, have been and are still being lowered. Finally, disease mongering is growing, because it is cheaper and faster to invent new "diseases" than new pharmaceutical drugs. From the definition of overdiagnosis it can be reasoned that a patient who has been correctly diagnosed and a person who has been overdiagnosed can have the same kind of pathologies. Therefore, at the level of the individual person or patient it can never be verified whether he or she has in fact been correctly diagnosed or overdiagnosed. Therefore, the complexity, dilemmas and pitfalls in understanding what overdiagnosis really is so succinctly captured by this quote from the Danish philosopher Søren Kirkegaard (1813-55): 'Life can only be understood backwards; but it must be lived forwards.