Before applying a screening program, health practitioners should assure the appropriateness of the factor understudy for screening. The disease or the risk factor appropriate for screening should be serious, highly prevalent although sometimes it is worth screening even for diseases of low prevalence which prove to have serious consequences as phenylketonuria. The natural history of the disease progression should be understood. Screening for asymptomatic diseases necessitates a long preclinical phase between the time when the disease can be first diagnosed by screening and the appearance of symptoms. Also, screening for risk factors, requires long exposure for disease development. Early intervention to remove exposure or treat the disease before appearance of symptoms should be known to be beneficial in terms of reducing morbidity and mortality than that given after symptoms develop.