As a water utility manager, your role in water security planning and threat response is critically important. This Water Security Handbook was developed by the U.S. EPA to help you, the water utility official, protect your water system and respond effectively to threats and contamination incidents involving your water system. EPA also wrote this Handbook in response to calls for a short, simplified document that summarizes the comprehensive document entitled 'Response Protocol Toolbox: Planning For and Responding to Drinking Water Contamination Threats and Incidents' (also known as the Response Protocol Toolbox). The Handbook is also intended to be a companion to EPA's 'Response Protocol Toolbox: Planning For and Responding to Drinking Water Contamination Threats and Incidents: Response Guidelines.' The 'Response Guidelines' provides many forms and checklists to help you organize and carry out your emergency response and planning efforts. This Handbook describes the basic concepts and procedures involved in water security planning and threat response. Together, the Handbook and 'Response Guidelines' provide succinct recommendations concerning water security planning and response actions.