The Amazon River Basin, which encompasses the world’s largest remaining tropical rainforest, has the highest diversity of fish species of any region in the world. This chapter reviews the main scenarios for environmental alterations in the Amazon Basin, which is predicted to be a function of global climatic changes and dams, identifies the potential impacts of different scenarios of environmental alterations in the Amazon Basin on Amazonian freshwater fish populations, identifies the consequences of the predicted impacts on the Amazonian freshwater fish populations, taking into account the main characteristics of the population dynamics and illustrates the potential social and economic consequences for the local and regional fisheries and the people who depend on these fisheries.