The northernmost extension of the Atlantic-wide overturning circulation consists of the flow of Atlantic Water through the Arctic Ocean. Two passages form the gateways for warm and saline Atlantic Water to the Arctic: the shallow Barents Sea and the Fram Strait which is the only deep connection between the Arctic and the World Ocean. The flows through both passages rejoin in the northern Kara Sea and continue in a boundary current along the Arctic Basin rim and ridges (Aagaard 1989; Rudels et al. 1994). In the Arctic, dramatic water mass conversions take place and the warm and saline Atlantic Water is modified by cooling, freezing and melting as well as by admixture of river run-off to become shallow Polar Water, ice and saline deep water. The return flow of these waters to the south through the Fram Strait and the Canadian Archipelago closes the Atlantic Water loop through the Arctic. © 2008 Springer Netherlands.