Sensitometric methods employed for determining the characteristic curve of interest in medical imaging are described. These methods are based on the way in which various recording systems are exposed. In diagnostic radiology, an x-ray intensity scale sensitometer is used for measurements of the characteristic curves of conventional film and screen-film systems. The effect of reciprocity law failure on the sensitometric evaluation of screen-film systems is illustrated. The shape of the characteristic curves as a function of the light spectrum emitted from three types of intensifying screens is presented. Sensitometric techniques for quality control of automatic processors are also discussed. In radiation therapy, a sensitometric technique is described for determination of the proper characteristic curve for a film which can be placed beneath the patient before radiation treatment and removed afterwards so that an image of the anatomy actually irradiated is obtained. A characteristic curve can also be obtained by means of which photographic densities measured on the therapy verification film is related to exit absorbed dose.