WaterSentinel (WS) is a program developed by the U.S. Environmental Protection Agency (EPA) in close partnership with drinking water utilities and other key stakeholders in response to Homeland Security Presidential Directive 9 (HSPD 9). The program involves designing, deploying, and evaluating a model contamination warning system (CWS) for drinking water security. A CWS is a system that collects information from a variety of sources, including monitoring and surveillance programs, in order to detect contamination events in drinking water early enough to reduce public health or economic consequences. This document presents an overview of the WS program and the CWS concept, including the design and development of the system architecture for the WS program, a framework for making design decisions, and considerations for evaluation of the WS program and the WS-CWS. In addition to guiding the design of the WS-CWS at the initial pilot utility, the approach described in this document can be used to inform other stakeholders and utilities interested in implementing a CWS. This is a living document that should evolve as experience is gained through the implementation of the WS-CWS, and this first version of the document represents the basis for the design of the first pilot.