A dynamical model for simulating and forecasting motion of sea ice fields in the Baltic Sea is presented. The model is based on the equations for conservation of mass and momentum and on an equation describing the distribution of ice mass. In the model, ice drift is caused by wind; currents beneath the boundary layer in the sea are neglected. The material behaviour of sea ice fields is described by a linear viscous law. Mass of sea ice fields is decomposed into concentration, level ice thickness, ridge density and ridge height. The model parameters have been estimated from the data from field experiments in the Baltic Sea. The equations are solved numerically with the finite difference technique; the grid size is 27 km and the time step 6 hours. The model has been in operational use in forecasting ice conditions in the Gulf of Bothnia since winter 1977; 30 h forecasts have been calculated daily and transmitted in chart form to the icebreakers.DOI: 10.1111/j.2153-3490.1981.tb01783.x