The aim of primary prevention is to reduce incidence. Secondary prevention is intended to lead to early detection of disease, and tertiary prevention aims to prevent reoccurrence of disease. The primary goal of screening is the early detection of disease in asymptomatic persons. Detecting disease at an early stage (lead time) does not necessarily imply an increase in life expectancy. Prior to the implementation of a mass screening programme, its effectiveness must have been demonstrated in randomised trials. Screening ought to be used for common relevant health problems, an accepted treatment method should be available and a recognisable latent or early stage of disease and the test to be applied should be acceptable to a broad population (WHO criteria). Examples include the ongoing national cancer screening programme and the field of occupational health and show that screening programmes do not always follow evidence-based criteria.