In April of 1993, Milwaukee, Wisconsin became the major focus of the drinking water industry. The focus of attention was centered on the increase in the number of reported cases of diarrheal patients throughout the city. The major increase in diarrhea was determined to be caused by the organism Cryptosporidium which was believed to be transported throughout the city via the drinking water. As a result of the outbreak, the City contacted the US EPA's Drinking Water Research Division (DWRD) for assistance. The DWRD sent a team of two engineers to Milwaukee. The team was to assist the city in determining how the Cryptosporidium may have passed through the drinking water treatment plant. The team conducted a rapid engineering assessment of the two treatment plants supplying the city with water. They also evaluated plant operational and laboratory data available for the time frame of concern. This paper discusses the efforts that the team took during the investigation. The recommendations that were made for improving the operation of the water treatment plants are also discussed.