In this presentation I provide an overview of some important biogeochemical processes affecting the behavior of metal(oid)s in contaminated peatlands, ranging from a global assessment to localized case studies at mine and smelter impacted sites in northern Ontario. The presentation will show that surface peat enrichment of many metals in peatlands is correlated with sulphur deposition highlighting the anthropogenic impact on peatlands globally. A notable exception is mercury (Hg), although the Hg/C ratio in peat is identical to the ratio identified in a global synthesis of surface waters. At the local scale metal contamination in peat decreases with distance from emission or contamination source, but porewater chemistry is highly variable, influenced by climate, but for many metals can be well predicted based on concentrations of metal in peat, pH and dissolved organic carbon (DOC). Contamination of peatlands can impact greenhouse gas emissions from severely contaminated peatlands, but this effect decreases with distance from contamination source. Emerging data showing the geochemistry of rare earth elements (REEs) in contaminated peatlands will also be presented.