This document outlines best practices for responding to a water supply contamination event. It emphasizes the importance of advance crisis preparation, activating a crisis management team, and effectively communicating with the public, regulators, and media. The first few hours and days of the response set the tone, so priorities include protecting public safety, investigating the issue, and treating or shutting down the affected water supply. Managing the various legal, reputational, and investigative aspects that follow requires a coordinated, strategic response. Overall, being ready before a crisis occurs is key to a successful outcome.