A comprehensive theory of radiographic density fluctuations is developed. The theory incorporates film granularity, quantum mottle, the absorbed x-ray photon energy distribution, fluctuations in phosphor light photon yield, and phosphor coating weight variations. The resulting expressions allow one to calculate the density fluctuations or, alternatively, the Wiener spectrum that will be obtained with a radiographic screen/film system from the physical parameters of the system. Reasonable agreement is obtained between experimental results and calculated values for three typical screen/film systems. An expression is also derived for the detective quantum efficiency (DQE) of screen/film systems, and the results of DQE calculations are presented for a commonly used screen/film combination as a function of density and of object size.