Six Arctic Ocean Model Intercomparison Project model simulations are compared with estimates of sea ice thickness derived from pan-Arctic satellite freeboard measurements (2004–2008); airborne electromagnetic measurements (2001–2009); ice draft data from moored instruments in Fram Strait, the Greenland Sea, and the Beaufort Sea (1992–2008) and from submarines (1975–2000); and drill hole data from the Arctic basin, Laptev, and East Siberian marginal seas (1982–1986) and coastal stations (1998–2009). Despite an assessment of six models that differ in numerical methods, resolution, domain, forcing, and boundary conditions, the models generally overestimate the thickness of measured ice thinner than ∼2 m and underestimate the thickness of ice measured thicker than about ∼2 m. In the regions of flat immobile landfast ice (shallow Siberian Seas with depths less than 25–30 m), the models generally overestimate both the total observed sea ice thickness and rates of September and October ice growth from observations by more than 4 times and more than one standard deviation, respectively. The models do not reproduce conditions of fast ice formation and growth. Instead, the modeled fast ice is replaced with pack ice which drifts, generating ridges of increasing ice thickness, in addition to thermodynamic ice growth. Considering all observational data sets, the better correlations and smaller differences from observations are from the Estimating the Circulation and Climate of the Ocean, Phase II and Pan-Arctic Ice Ocean Modeling and Assimilation System models.