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Screening identifies possible unrecognized disease using rapid tests and examinations. Full diagnosis is then done on identified cases. Screening methods should be sensitive (effective in detecting the given disease - free of false-negatives), specific (give results free of false-positives) and acceptable (minimal discomfort so that people are not discouraged from taking them). To be successful screening programs should target a common type of cancer with high morbidity and mortality. Effective treatment should also be available. There is no point in detecting cases that cannot be effectively treated. The test itself should be acceptable, safe, relatively inexpensive, and efficiently administered, and subject to a quality control system. Policies should be in place to define the frequency of screening and the ages at which it will be done. An information system with good records is required to assure follow-up of positives so that they may be diagnosed and treated quickly and appropriately. The system sends out invitations for initial screening and recalls individuals for repeat screening. It then follows those who test positive. It also monitors and evaluates the program. A large portion (80%) of the target population should be screened, and should be defined to include all those at high risk. But screening that concentrates only on high risk groups is rarely justified since such groups are only a small proportion of a country’s cancer burden. Countries vary in how they carry out screening. Some use organized screening programs while others use cancer detection as a part of routine medical practice. Yet others just encourage people to get specific tests at regular intervals. But the research shows that only organized mass screening programs significantly impact national cancer burden. Examples of screening tests include the PAP smear for cervical cancer; radiological screening (breast and stomach); endoscopy (colon); chemical tests (stool occult blood); or blood tests like alpha-fetoprotein (liver), CA 129 (ovary) or prostate-specific antigen (PSA for prostate).