This 26-minute video uses a computer simulation to demonstrate how x-ray photons interact in the body through coherent scattering, the photoelectric effect, Compton scattering, and pair production. It emphasizes the statistical nature of photon attenuation and energy absorption. The viewer should be able to distinguish between the quantities energy transferred, energy absorbed, Kerma, and absorbed dose, describe the effect of secondary photons on energy transferred and absorbed dose, and understand the effect of photons of different energy when used for radiation therapy.