We combine results from hydrographic measurements and numerical models to investigate the circulation in the Arctic Ocean. Observations exhibit a marked difference in the temperature and salinity distribution west and east of the St. Anna Trough. West of the St. Anna Trough the warm and saline core of the Fram Strait branch is found close to the shelf break. East of the St. Anna Trough the highest temperatures and salinities are found further off shore. A colder, relatively fresh layer of Barents Sea outflow displaces the Fram Strait branch. The numerical model reproduces these hydrographic structures. It depicts the Barents Sea branch as the most important inflow into the Arctic with respect to heat, fresh water, and volume. The divergence of the heat transport in the Barents Sea is as great as the net heat transport into the Arctic Ocean. A large part of the freshwater input takes place in the area of the Barents/Kara Sea. The model transports associated with the Fram Strait branch are smaller because most of the Atlantic water recirculates in Fram Strait. Bottom topography plays a crucial role for the model circulation. Our results support previous conceptual models of the Atlantic layer circulation in the Arctic Ocean. The model circulation is mainly thermohaline driven. Ventilation times for the Eurasian and Canadian Basins below 1-km depth are estimated as 220 and 420 years, respectively. ¿ 1997 American Geophysical Union