Screening can be a very powerful tool for prevention or more effective treatment of diseases. However, a number of prerequisites have to be met. Only diseases with a preclinical phase, during which the disease or its precursors can be detected by a suited test, are amenable to screening. Early detection of the disease must enable either prevention or more effective management of the disease and not just prolong the "patient career". The benefits of screening must encompass potential harms, which may include, for example, complications, false positive diagnoses or over-diagnoses (i.e. the diagnosis of clinically irrelevant disease). Benefits from screening must be achieved at acceptable costs. Implementation of screening has to be based on scientific evidence and accompanied by scientific evaluation.