A model for the sensitivity of x-ray film in the 1–100-keV region is presented. It provides the only known method for the interpretation of densities due to polychromatic exposures in the low-energy (1–5 keV) region. A computer program utilizing the model has been found to be a convenient and flexible tool for predicting and interpreting low- and high-energy film densities. The predicted densities were found to agree with experimental data for Kodak No-Screen film with an error of less than 20%. The model is a schematic representation of the physical processes which take place during x-ray transport through the emulsion, the production of developable grains, development, and densitometry. It contains an allowance for the different fractions of scattered light collected by different densitometers. In the low-energy region it handles x-ray absorption using a theory for absorption in heterogeneous media. In the high-energy region an enhanced sensitivity due to the effect of photoelectrons is included.