Summary The geographical scope of the streams and rivers of the Arctic is primarily delineated byte “Arctic Ocean Watershed,” or the area of land from which freshwater drains to the Arctic Ocean. Considering that there are many other rivers and countless streams draining the Arctic Ocean Watershed, it is clear that its runoff is a significant component of global water and element cycles. The fact that these major rivers integrate both boreal and arctic landscapes has a large influence on their physical and ecological attributes, making them difficult to characterize from a regional perspective. These strictly arctic ecosystems are the focus of this chapter, although features of the larger, 'arctic-boreal' rivers will be addressed where relevant. The 16 million km2 Arctic Ocean Watershed contains four of the Earth's largest rivers. The watersheds of these rivers integrate both boreal and arctic landscapes, making them difficult to characterize regionally.