It is widely acceptable the principle which suggests that prevention is better than cure and screening is a major strategy of secondary prevention. According to UK National Screening Committee (1998) screening could be defined as: the systematic application of a test or enquiry, to identify individuals at sufficient risk of a specific disorder to warrant further investigation or direct preventive action, amongst persons who have not sought medical attention on account of symptoms of that disorder. According to guidelines of WHO and of other professional bodies screening programs should be based on principles concerning the viability, effectiveness and appropriateness of a screening program. Although it is considered that the screening tests that are used in present follow the proposed principles it has been proved that the screening tests carry not only benefits but also risks. The major risk of screening test is the proportion of the false positive and the false negative results, because of their medical, psychological, ethical and legal consequences. Screening for Down syndrome, which now is widely practiced, it can be used as an example that reflects the medical, psychological, economical and ethical implications of screening. For example, the medical con-sequences of a false positive result in the case of antenatal screening for Down syndrome may include unnecessary invasive procedures (amniocentecis, chorionic villus sampling), the physical consequences of them and also adverse outcome of the pregnancy. Although screening has a great contribution in reduction of mortality and morbidity it must be also considered that screening have many implications and consequences. Therefore the informed consent of individuals towards screening is necessary.