Guide for the Selection of Commercial Explosives Detection Systems for Law Enforcement Applications (NIJ Guide 100-99) provides law enforcement agencies with information to be used in the selection and utilization of explosives detection equipment and techniques. This NIJ guide considers a wide array of important factors that should be considered by purchasers, including cost, sensitivity, portability, and ease of use. Advice about what technologies are likely to work best in various applications and a market survey of trace and x-ray-based commercial detection systems known to the authors as of October 1998 are also included.