Peatlands are common in montane areas above 1,000 m in New Guinea and become extensive above 3,000 m in the subalpine zone. In the montane mires, swamp forests and grass or sedge fens predominate on swampy alley bottoms. These mires may be 4–8 m in depth and up to 30,000 years in age. In Papua New Guinea (PNG) there is about 2,250 km2 of montane peatland, and Papua Province (the Indonesian western half of the island) probably contains much more. Above 3,000 m, peat soils form under blanket bog on slopes as well as on valley floors. Vegetation types include cushion bog, grass bog and sedge fen. Typical peat depths are 0.5‒1 m on slopes, but valley floors and hollows contain up to 10 m of peat. The estimated total extent of mountain peatland is 14,800 km2 with 5,965 km2 in PNG and about 8,800 km2 in Papua Province. The stratigraphy, age structure and vegetation histories of 45 peatland or organic limnic sites above 750 m have been investigated since 1965. These record major vegetation shifts at 28,000, 17,000‒14,000 and 9,000 years ago and a variable history of human disturbance from 14,000 years ago with extensive clearance by the mid-Holocene at some sites. While montane peatlands were important agricultural centres in the Holocene, the introduction of new dryland crops has resulted in the abandonment of some peatlands in the last few centuries. Despite several decades of research, detailed knowledge of the mountain peatlands is poor and this is an obstacle to scientific management.