How to weigh the significance of each air bubble when conducting an examination of these footwear impressions is discussed. The most common methods of manufacture of polyurethane outsoles and how each of these methods affect the resulting air bubble pattern are explained. A summary of the chemical, mechanical, and physical variables influencing the position and contour of these trapped air bubbles is given. These characteristics, alone or in conjunction with the traditional wear patterns and accidental characteristics acquired during the use of the shoe, can help the examiner form a stronger opinion toward the identification of the footwear. (Author abstract)