The history of passive treatment of acid rock drainage dates back over 20 years. It is only recently that engineers and scientists have been able to discern how Mother Nature has been immobilizing metals in natural wetlands and to mimic her handiwork. Since 1988 (when engineers and scientists gathered at two major technical conferences in Pittsburgh and Chattanooga), the geochemistry of metal precipitation in oxidizing and reducing environments has become better understood and the capacity of passive treatment systems for mine drainage has reached levels of 1,200 gpm. Systems operating in tropical and alpine environments indicate that this technology has broad application. While there have been advances, a "cook book" approach to design has yet to be realized. However, a staged design protocol of laboratory, bench-, and pilot-scale testing has yielded full-scale designs that have been functioning as intended. Future advancements needed include a focus on sulfate removal and the recovery of resources that might make this already economical water treatment method even more so.