Abstract. The Zortman Landusky mines in Montana, USA, produced gold and silver from a mineralized syenite intrusion. Although mining in the area began over 100 years ago, the most extensive production was from open pit mining and heap leach cyanide processing that occurred from 1977 until 1998. Zortman Landusky is where valley-fill-heap-leach cyanide processing was first used for gold production and is the first large-scale gold mine where unexpected consequences of acid rock drainage occurred. Neither the mining industry nor the agencies anticipated the problems, which developed at the mine. Subsequently Zortman/Landusky is where many of the best management practices for mining and new reclamation techniques were developed. In 1998, the operator declared bankruptcy and the site was taken over by the U.S. Bureau of Land Management and the Montana Department of Environmental Quality. This paper details the history of acid rock drainage issues at the site from the first recognition that acidic drainage was a problem, through initial characterization and prediction work, to final reclamation and water treatment.Closure costs to date include approximately US$42M for site reclamation and approximately US$15M for water treatment facilities. Work continues on residual impacts many of which were not fully recognized until well into the closure phase. These include characterization and treatment strategies for acid mine drainage in Swift Gulch, a small stream whose headwaters originate on the mine property and which eventually flows onto adjacent tribal lands Some of the lessons that Zortman/Landusky are that detailed characterization and closure planning before and throughout the mine life are critical. This project also highlights the importance of having adequate financial guarantee mechanisms available to support both the anticipated reclamation throughout the project life and any potential water treatment.