This paper describes a serious water contamination incident which took place in May 2000 in Walkerton, Canada. This is a town of about 5,000 people located about 175 km northwest of Toronto. Seven people died and several thousand became ill as a result of this incident. Because of the seriousness of the event the provincial government established a Judicial Inquiry to look at both the causes and responsibility for the incident and to examine broader questions relating to drinking water in the province. The authors of this paper were retained by the Commission of Inquiry to assist in the establishment of physical cause. This paper discusses water treatment and distribution aspects of the situation, drawing on the authors' presentations and assistance to the Inquiry. The determination of cause is the purview of the Commissioner, Mr. Justice Dennis O'Connor. His report on cause is expected to be published early in 2002. Includes 18 references, tables, figures.