Methods of forecasting sea and swell in the open ocean have been given by Sverdrup and Munk and others. In this paper are presented principles from which techniques for forecasting sea and swell in a broken icefield may be developed. It is shown that water waves entering an icefield are damped exponentially with increasing ice thicknesses. It is further shown that selective damping occurs in the icefield, the shorter‐period waves being more easily damped. Graphs are included which help to give a clearer picture of the result