In many mining regions of the world, pollution of surface water and groundwater by drainage water originating from mines aiming waste poses either a serious threat to the environment, or a severe environmental problem. During the last two and a half decades, treatment of mine drainage water in constructed and natural wetlands has emerged as an alternative to more conventional methods to handle the problem. In this thesis, the major biogeochemical processes behind metal immobilization in wetlands are summarized. Factors that influence the efficiency and longevity of these processes are discussed based on a review of previous experiences from wetlands exposed to mine drainage waters. The potential for successful treatment is largely determined by the characteristics of the drainage, the morphology of the wetland, and the degree of maintenance planned. In maintenance-free wetland, factors that have to be considered include: changes in drainage water production and wetland performance over the years, the total metal accumulating capacity of the wetland, and the post treatment integrity of the wetland. Results from a case study indicated that no or little immobilization of metals occured in natural wetlands situated along a mining region recipient (the river Vormbäcken, northern Sweden). However, Fe supplied from the catchment area appeared to favor the fraction of As, Cu, and Pb recovered in particles, a mechanism that could be of interest for the polishing of treated mine drainage waters, especially when combined with settling in a downstream wetland. Laboratory experiments showed that such a process is likely to be favored by addition of Fe in its ferrous form, higher water temperatures, presence of Ca, and absence of dissolved organic matter.