The environmental, social, and economic problems associated with acid mine drainage (AMD) from abandoned mine sites are globally significant. As a consequence of mining exploration of metallic mineral deposits, large quantities of sulfides are exposed to weathering processes (oxygen, water, and bacteria) that generate acidic environments and promote metal(loid) solubilization, forming so-called AMD. The pollution caused by AMD can be minimized by adopting suitable strategies and approaches to mitigate AMD or prevent its generation and migration. The current work was undertaken for an in-depth account of the generalized features of AMD and in-depth case studies of the impact of AMD in the northern part of Portugal. The first part was prepared to highlight basic features dealing with (the) origin of AMD; (2) the characteristics of AMD; (3) factors controlling the formation of AMD; (4) the environmental impact of AMD; (5) resource recovery from AMD; and (6) prevention, mitigation, and treatment of AMD. The second part accentuates in-depth case studies of mine drainage water collected bimonthly from abandoned mines situated in northern Portugal over 1 year. In the mines that were studied are acidic waters with low pH and significant concentrations of SO4, and metal(loid)s (As, Fe, Mn, Zn, Cu, Pb, Cd, and Ni), whereas in superficial natural stream waters outside the mines, the pH is close to neutral, with low conductivity and lower metal concentrations. Stream waters inside the mines have an intermediate composition between AMD and natural stream waters outside the mines. The environmental impact of the quality of the superficial waters is marked only in the closest surroundings of the pollution source (1 to 2 km). Several coherent and integrated processes contribute to this situation, such as (1) dilution by other groundwater and surface waters that flow toward stream waters; (2) precipitation or co-precipitation of metallic cations by hydroxides and sulfates; and (3) adsorption of metallic cations by organic and inorganic sediments and aquatic plants. Hydrochemical processes and the environmental impact in the study sites are important issues necessitating potential resource recovery from AMD (eg, iron or other metals) and appropriate strategies and approaches to mitigate AMD and pollution prevention.