The analysis and understanding of results of computed tomography (CT) require an understanding of photon attenuation in matter. The high sensitivity and resolution of these devices coupled with the use of a polychromatic photon source require a level and breadth of understanding about photon attenuation not usually required in any particular subspecialty of radiological physics. With this goal in mind, a discussion of narrow-beam photon attenuation in matter is given and related to those problems currently underway in the field of computed tomography. Measurements and calculations of tissue properties are presented. Calculations of descriptive quantities relevant to polychromatic source attenuation and CT scanning are described and presented.