Identification of previously unrecognized disease or a disease precursor, using procedures or tests that can be conducted rapidly and economically on large numbers of people with the aim of sorting them into those who may have the condition(s) for which the screening procedures have been carried out and those who are free from evidence of the condition(s). The UK Screening Committee defines screening as a “public health service in which members of a defined population, who do not necessarily perceive they are at risk of, or are already affected by, a disease or its complications, are asked a question or offered a test, to identify those individuals who are more likely to be helped than harmed by further tests or treatment to reduce the risk of a disease or its complications.” When screening for noncommunicable conditions, the benefits for the individual should exceed the costs or risks. With communicable diseases such as tuberculosis or chlamydia, the primary aim is to protect population health by preventing transmission of the infectious pathogen to susceptible persons, but of course the infected individual benefits too. Screening may be conducted for a single condition, e.g., mass miniature radiography screening to detect pulmonary tuberculosis, or may consist of a battery of tests, as in multiple or multiphasic screening programs. Much information about screening is available at several Web sites. See, for example, http://www.cancerscreening.nhs.uk/http://www.nsc.nhs.uk/ and http://www.ahrq.gov/clinic/cps3dix.htm. See also sensitivity, specificity, and predictive value.