The basis for footwear impression evidence is the determination of the source of a footwear impression recovered from a crime scene. The process of examining footwear impression evidence takes into account class and identifying characteristics. Class characteristics result from the manufacturing process, such as physical size, design, and mold characteristics. Identifying characteristics, on the other hand, do not result from the manufacturing process, but are accidental, unpredictable characteristics that result from the wearing of the footwear. This can include the impression of objects that have become attached to the outsole or marks on the outsole caused by cuts, nicks, gouges, and scratches. A match between the class characteristics of the footwear that made an impression at a crime scene and a known shoe can only conclude that the known shoe "could have made" the questioned footwear impression. Identifying characteristics and their orientation and placement on the outsole, however, provide a more convincing correspondence between a known shoe and the footwear impression at the crime scene, even though additional identifying marks may have been made on the known shoe since the time of the crime. Some researchers (Stone, 2006) have developed mathematic models for predicting the frequency of accidental common characteristics of footwear impressions and a known shoe. These models support the extreme rarity of such accidental identical features. The paper's section on validation contains mathematical formulas that can be used to determine the probability that the features of a given footwear impression at a crime scene were made by a known shoe. 24 references