The Environmental Protection Agency (EPA) has implemented all of the recommendations GAO made in its May 2011 report to improve the Unregulated Contaminant Monitoring Rule (UCMR) program. EPA's UCMR program collects data on unregulated contaminants in the nation's drinking water. EPA uses this data and other information to make decisions on whether to regulate additional drinking water contaminants. This book examines the extent to which EPA implemented GAO's prior recommendations to improve the program and opportunities, if any, to strengthen it further; the factors EPA considered when it selected the UCMR3 contaminants and the limitations, if any, it faced in selecting them; and the extent to which UCMR data support regulatory determinations. It also discusses and summarises the Safe Drinking Water Act (SDWA) and its major programs and regulatory requirements.