Water quality deterioration in distribution systems, mainly caused by inappropriate planning, design and construction or inadequate operation and maintenance and water quality control, has been linked to a significant proportion of the burden of waterborne and water-related illness. Stresses on these systems caused by rapid urbanization, population growth and aging infrastructure further exacerbates the problems. This reference tool has been developed to help water suppliers and regulators who are familiar with the Water Safety Plan approach, enhance their risk assessment and management and investment planning for their water distribution systems.