The use of observation wells with slug tests would be of relatively little interest if the sole purpose of the wells was to demonstrate the size of the affected volume. Several methods have been proposed for the analysis of response data from multiwell slug tests, but few have seen much use in field settings. Traditionally, slug tests have been performed using a single well as both the site of the stress and the site at which head measurements are taken. The actual volume affected by a slug test is difficult to define. Most workers have used a definition based on the distance some arbitrary normalized head will propagate during a test. If the impact of inertial mechanisms and/or wellbore storage at the observation well cannot be ignored, the Butler and Zhan model should be used to analyze the response data.