This book on the problem of acid waters in the Iberian Pyrite Belt (FPI) is structured in 7 chapters, including its most relevant conclusions at the end of each one. After the introduction, the basic notions about the processes that produce acid mine drainage (AMD) are described, necessary to understand the characteristics of this type of pollution (Chap. 2), which intensely affects river basins. Tinto and Odiel mainly due to mining from 1850 to the end of the 20th century (Chapters 3 and 4). The fifth chapter is dedicated to the impact on the reservoirs of the entire Spanish part of the FPI, whose acidity depends on the balance between acid inputs and the flow of waters not affected by AMD that they receive. The Tinto and Odiel rivers maintain their acid character until their mouth in the estuary of the Ría de Huelva, where a distribution of contaminants occurs between the water column and the sediment, which causes an impact on the biota of the transition waters ( Ch. 6). As a solution to the problem, the treatment techniques for these acid leachates from sulfide mining are presented, and strategies and measures that could be applied in the Odiel basin to improve its condition are recommended (Chap. 7).