Abstract Screening programs for various medical and psy-chological difficulties are becoming more prevalent. Their appeal is obvious: problems that are caught early should be easier to treat and do not lead to secondary and tertiary sequelae. However, there are a number of problems that may be associated with mass screening. These include the psychometric properties of the tests themselves, which may not have adequate sensitivity and specificity. Second, even if the tests do meet minimal criteria for va-lidity, the low prevalence of many disorders means that the majority of cases that are identified will actually be false positives. This in turn leads to additional financial costs, because of the added work load and the necessity of deter-mining who are not cases as well as psychological costs due to labeling. The final difficulty is that the effectiveness of the interventions needs to be evidence-based. Recommendations are made regarding when screening programs should be implemented. Keywords Screening.Prevalence. Lead time bias.Length bias.Sensitivity.Specificity