The Process Optimization Guidance (POG) summarizes mercury emission reduction practices applicable to coal-fired power plants. Mercury emissions from combustion of coal in power plants and industrial boilers constituted approximately 26 per cent of the global anthropogenic emissions of mercury in 2005. Therefore, it is important to decrease the amount of mercury emissions from this sector. The POG is a tool to help determine the approaches to control mercury emissions, in many cases a co-benefit of reducing emissions of other pollutants. It is a tool for individual coal-fired power plants. The POG allows for a preliminary selection of a mercury control strategy for a given power plant. In general, the POG is also applicable to coal-fired large industrial boilers. The practices and technologies capable of reducing mercury emissions include energy efficiency improvement measures, pre-combustion control measures (e.g., coal blending, coal cleaning), optimizing other (single) air pollutants control technologies to maximize mercury removal, mercury-specific control technologies, and multipollutant control technologies.