Description: Arctic sea ice extent and thickness have undergone dramatic changes in the past decades: Summer sea ice extent has declined at an annual rate of approximately 12.7 % per decade over the satellite record (1978 – present, [5]) and its mean thickness has decreased by 0.58 m +/- 0.07 m per decade over the period 2000 - 2012 [3]. The thinning of sea ice is accompanied by an increase of ice drift velocity [8], deformation [7] and a decrease of net ice growth rates. Climate model simulations indicate that ice extent and thickness will further decline through the 21st century in response to atmospheric greenhouse gas increases. However, the mass balance of Arctic sea ice is not only determined by changes in the energy balance of the coupled ice-ocean-atmosphere system but also by the increasing influence of dynamic effects. One aspect of the mass balance of Arctic sea ice are changes of ice volume export rates through Fram Strait and the decline of thick and old multi-year ice North of Ellesmere Island. Thickness surveys carried out north of Greenland and Fram Strait give insight into composition and properties of Arctic sea ice in general and how it changes over time. An extensive data set of ground-based and airborne electromagnetic ice thickness measurements were collected between 2001 and 2017 during several aircraft (PAMARCMIP, TIFAX) and Polarstern campaigns. The first aim of the IceBird 2018 summer campaign is to complement earlier measurements made north of Svalbard, Greenland and in Fram Strait. Sea ice thickness information will be used to examine the connection between thickness variability, ice age and source area. Together with satellite based information on sea ice motion, data will be used to quantify sea ice outflow through Fram Strait in summer. These estimates shall improve the understanding of interannual variability in summer sea ice outflow and complement existing winter volume flux calculations. A second objective is to extent sea ice thickness measurements to the Lincoln Sea where we will study thinning of sea ice due to reduction of old multi-year ice in this area. Like the measurements planned over the Fram Strait area, the surveys are a continuation of earlier aircraft campaigns made north of Alert and shall improve understanding of ice mass balance changes in the Arctic.