A biological warfare agent (BWA) is a microorganism, or a toxin derived from a living organism, that causes disease in humans, plants, or animals or that causes the deterioration of material. The effectiveness of a BWA is greatly reduced if the attack is detected in time for the target population to take appropriate defensive measures. Therefore, the ability to detect a BWA, in particular to detect it before the target population is exposed, will be a valuable asset to defense against biological attacks. The ideal detection system will have quick response and be able to detect a threat plume at a distance from the target population. The development of reliable biological standoff detection systems, therefore, is a key goal. However, testing biological standoff detection systems is difficult because open-air field tests with BWAs are not permitted under international conventions and because the wide variety of environments in which detectors might be used may affect their performance. This book explores the question of how to determine whether or not a biological standoff detection system fulfills its mission reliably if we cannot conduct open-air field tests with live BWAs.