The importance of the Amazon area to sustain the global equilibrium in the environment has been recognized worldwide. This has been much more accentuated in the present days due to the intense debate related to global warming. Consequently, all initiatives/studies directed to a better knowledge/management of that huge environment are welcome and needed. This book is a contribution to this task, as gold has been exploited intensively in the Brazilian Amazon during the past 30 years using garimpo methods (small-scale gold mining), where the elemental mercury (Hg) used in amalgamating the gold, the final stage of the ore dressing process, has caused abnormal Hg concentrations in waterways. This has occurred in several areas of the Amazon region, where most of the ore prospected is alluvial. Particular attention to the Madeira River has been given since 1986 by several investigators. The main reason for this is that the Madeira River is the largest tributary of the Amazon River and the gold mining was officially allowed on a 350-km sector of the river, for its mid and upper reach, in the northwestern reach of the Amazon basin. Consequently, mercury was released from gold-mining fields to the atmosphere or to waterways in the metallic form, due to the large number of mechanical dredges operating simultaneously (about 6,000 during the peak mining activities). Although Hg0 is relatively immobile in the aquatic environment and its solubility is low in water, Hg contamination in people living upstream and downstream from garimpos has been reported. The gold-mining activities on the Madeira River basin reduced substantially in the present days, i.e. it is practically absent. However, despite this, it is necessary a better understanding of the Hg behavior in tropical aquatic systems, mainly close to the most populated areas, as people may be still suffering toxicological consequences of the Hg releases in the past. Therefore, even in the present days, the knowledge of the mercury occurring in the aquatic system of the Madeira River basin is a great concern by local/international authorities and environmentalists, since it can contribute for identifying the effects of the anthropogenic Hg inputs relatively to the background reference levels expressing the natural Hg concentration. This book describes the results obtained on the analysis of samples of water, bottom sediments, suspended solids and fishes that were collected at the Madeira River basin, Brazil, with the purpose of investigating the mercury release in the aquatic environment as a consequence of the gold mining activities.