Drift ice material -- Ice kinematics -- Sea ice rheology -- Equation of drift ice motion -- Free drift -- Drift in the presence of internal friction -- Numerical modelling -- Use and need for knowledge on ice drift -- Study problems -- References.The author presents in The Drift of Sea Ice the fundamental laws of sea ice drift which come from the material properties of sea ice and the basic laws of mechanics. The resulting system of equations is analysed for the general properties of sea ice drift, the free drift model and analytical models for ice drift in the presence of internal friction, and the construction of numerical ice drift models is detailed. The science of sea ice drift through its 100-year history to the present day is explained. The text includes the geophysical theory, observations from field programs, and mathematical models. Topics covered include the science of sea ice drift, forecasting drift velocity based on volume, size and shape, sea ice ridging and remote sensing, modelling of the ice conditions, and finally the role of sea ice drift in research fields in ice-covered seas, oceanography, marine ecology and engineering.