The Metropolitan Water Reclamation District of Greater Chicago is responsible for all of the regional stormwater management for Cook County in northeastern Illinois, one of the largest urban areas in the United States. Continuing urban expansion in this area has increased stormwater runoff and combined sewer overflows and likely decreased groundwater recharge. Passive induced-infiltration structures may help reduce stormflow problems and increase infiltration. These structures must be properly located to function effectively. Using hydrogeologic and land cover and use characteristics, maps of Cook County were developed indicating areas having the most potential for inducing stormwater infiltration. This assessment is preliminary because the scale of the mapping only gives a general indication of potential infiltration areas and is not suitable for site-specific investigations. In Cook County, 76,080 of 612,636 acres (12.4 percent) were determined to have the greatest potential for passive induced-infiltration-structure locations. Of these 76,080 acres, 8,650 are within the Lake Michigan surface watershed, with the remaining 67,430 acres within the Illinois River surface watershed. If all the annual rainfall on the 8,650 acres infiltrated and flowed to Lake Michigan, the resulting groundwater flux would be about 33 cubic feet per second (ft3/s).