This chapter provides an overview on the range of hydrologic extremes occurring in Arctic Rivers, consisting of extreme low winter flows; river-ice jam breakup spring floods; snowmelt-driven peak spring/early summer flows; and in some instances, rainfall-driven peak flows in summer. These extreme conditions are mainly influenced by climatological drivers, and in particular, warming climate and enhanced wetness is causing substantial changes in the magnitude, variability and timing of extreme events. The most prominent historical changes in the Arctic include increasing trends in mean annual flow and winter low flow, and earlier timing of peak flow, which are attributable to warming temperature and increasing precipitation, and resulting changes in snowpack storage. Winter low flow is further enhanced by permafrost degradation as it promotes increased soil infiltration and subsurface water movement. Snowmelt-driven annual maximum flow, primarily, has been decreasing, consistent with increased warming and decreased snowpack. Secondary peak flow events in late summer, driven by extreme summer rainfall and possibly enhanced by glacial melt, have been occurring more frequently in some areas of the Arctic, exceeding snowmelt-driven peak flow events. There is also evidence of nonstationary changes in streamflow extremes, such as increasing recurrence intervals of snowmelt-driven floods of a particular magnitude. Future projections indicate continued and enhanced warming, and strong increases in the high-latitude precipitation leading to enhanced annual flows and low flows. Future changes in peak flow remain unclear as peak flow could either increase or decrease depending on the region and interactions between precipitation change and temperature increases. Alterations in hydrologic extremes in the Arctic will have major social and economic implications; thus, focused research aimed at understanding, predicting, and projecting the Arctic streamflow extremes is recommended.