This guide has been prepared to help employers, workers, members of joint health and safety committees, supervisors, and occupational health and safety personnel meet the requirements of the Ontario designated substances regulation regarding mercury in the workplace, and to help them understand the responsibilities that this regulation places on all participants in the workplace health and safety system. Sections of the guide cover the hazards of mercury in the workplace, including what mercury is, how it is used, health hazards, and chemical reaction hazards; the mercury regulation, including who is covered and the allowable airborne concentrations of mercury; assessment and control of mercury exposure, including spill cleanup, protective clothing, workplace design, and air monitoring; and medical surveillance for exposure to mercury, including clinical tests, sample analysis, and physician reporting protocols. Appendices include information on first aid and on mercury spill cleanup.