This chapter provides two case studies of permeable reactive barriers (PRBs) installed to intercept and treat groundwater affected by acid drainage and heavy metals. The Nickel Rim permeable barrier was designed to treat effluent containing potential acidity (as dissolved ferrous iron) from decommissioned mine wastes, and the Vancouver barrier was designed to treat groundwater impacted by elevated concentrations of heavy metals, including cadmium, copper, nickel, and zinc. Both reactive barriers treated the impacted groundwater to acceptable concentration levels while decreasing the net acidity within the groundwater. Sulfate reduction rates within both barriers decreased with time, however, rates were sufficient to attenuate heavy metals below regulatory guidelines. In order to design and install an effective PRB, the geology, hydrogeology, and geochemistry of the aquifer system must be well characterized and understood. Design specifications should account for heterogeneities within the aquifer and barrier as well as the effects of fluctuating water tables and the possibility of reductive dissolution of ferric iron (oxy)hydroxide phases within the aquifer downgradient of the reactive barrier. Permeable reactive barriers provide a cost-effective strategy for the long-term treatment of acid mine drainage and heavy metals in both urban and remote settings.