This thesis is concerned with the modelling of sea ice, particularly in regions where it is composed of individual floes interacting through collisions. This has been done by modifying and extending existing models that have demonstrated their ability to simulate sea ice in various Arctic and Antarctic regions. The purpose of this study is the introduction of the representation of floes, in terms of their size and number, into a sea ice model, thus adding a feedback mechanism and a further output to the output fields normally produced by sea ice models, the ice velocity and the ice thickness distribution and the ice concen tration. Many of the physical processes concerning floes that are relevant to a sea ice model have not yet been investigated quantitatively. These aspects of floe behaviour used as model input are calculated from idealized mechanical models of a floe field. These include determinations of floe collision rates, side melting of floes and the cracking of floes in high winds. The strength of the pack ice is investigated, and in particular the effect of open water on the strength is considered. The shape of a plastic yield curve used in the model to determine the ice interaction forces is derived theoretically. The model used includes both thermodynamics and dynamics. The ice thickness characteristics and floe sizes change due to growing and melting, advection, floe cracking, floe collisions, and redistribution processes such as ridging and rafting. Daily wind and temperature data together with long term ocean currents are used as input to drive the model. The results of a six month simulation of the sea ice development in the (Eas t) Greenland region are presented and discussed together with a comparison with the observations.