The Hazardous Substances Emergency Events Surveillance (HSEES) system is a comprehensive, state-based surveillance system of hazardous substance releases and public health consequences. Maintained by the Agency for Toxic Substances and Disease Registry (ATSDR) since 1990, the system captures information on acute releases of hazardous substances that need to be cleaned up or neutralized according to federal, state, or local law. Information about threatened releases that result in public health action such as evacuation is also included. Of the 39,766 events reported to HSEES for 1996--2001, 8% resulted in deaths or injuries. Funded through a competitive program announcement, 15 states currently participate in HSEES. State coordinators actively collect data from multiple sources after an eligible event occurs and enter data about the event into a standardized ATSDR-provided web-based system. The information in HSEES describes the distribution and characteristics of hazardous substances emergencies and the morbidity and mortality experienced by employees, responders, and the general public as the result of hazardous substances releases. Analysis of HSEES data helps identify risk factors associated with hazardous substances releases. For example, although events in which chlorine was released account for only 1.6% of all events, they were 3.52 times more likely to result in injuries. Knowledge of these factors is useful in planning public safety interventions and can impact the formulation of guidelines and policies to help reduce the number of events (primary prevention) and the morbidity and mortality associated with these events (secondary prevention). Utilizing state-specific analyses of HSEES data, participating states have been able to develop prevention outreach activities such as awareness training of first responders, primary prevention of spills, and secondary prevention of related injuries and deaths caused by ammonia, chlorine, and mercury. Specific examples involving ammonia, chlorine, and mercury releases will be presented in detail.