Computed tomography (CT) examinations are increasingly used for clinical diagnostic and research purposes, as they provide in vivo anatomic information similar to that provided by gross anatomy. In conjunction with physiologic maneuvers or contrast media, CT may also provide in vivo physiologic information. Using calibrated acquisition protocols, accurate noninvasive measurements of tissue density, air volume, blood volume, and capillary perfusion can be performed. Serial CT scans can provide longitudinal measurements indicative of disease progression or regression, allowing noninvasive assessment of treatment effects. However, the X-ray radiation associated with CT has been associated with a small but significant increased risk of malignancy, which may be fatal. Large studies have detected this small risk, which appears to be related to the cumulative radiation dose of all previous exposures in a linear fashion. It has been shown that the risk from a given radiation exposure is greater in young people and females compared with older males. The combination of these two risk-enhancing factors, found in pregnant females, provides the greatest risk. Radiation risk decreases with increasing age for both men and women, asymptotically approaching zero. Radiation risk can be calculated using dose metrics provided on current CT scanners as outlined in this article. Ethically, given that radiation is associated with measurable risk, clinically indicated and research CT examinations must provide an increase in knowledge that has substantial benefit to the subject. This benefit should be related to the potential of saving of life or to the prevention or mitigation of serious disease.