The reduction of mercury emissions from fossil fired power plants is of major concern. The optimization and the improvement of the existing flue gas cleaning devices or the use of special equipment for mercury removal are possible ways. Therefore the use of sorbents, the optimization of SCR-DeNOx-catalysts and the utilization of additive to enhance the mercury oxidation and the separation in wet flue gas desulfurization are only some discussed technologies. As the behavior of all mercury species in flue gas depends on the operational parameters of the power plant and the special flue gas cleaning device, it is essential to understand the chemistry and the interaction of the different cleaning devices.