The breakup of the winter ice cover is a brief but very important event in the regime of rivers. The abrupt change from a condition of complete and continuous ice cover to one of the alternating reaches of ice jams and open water, followed by severe surges and ice runs with rapidly rising water levels and temperatures, is a major significance to the river and basin ecosystems. Changes to the pattern and timing of breakup, due to natural or anthropogenic factors, can be highly detrimental to these ecosystems, though beneficial effects are also possible. Moreover, the breakup is often a time of major ice jams that cause serious flood damages and even loss of human life, channel and bank erosion, or they seriously impede navigation. Ensuring that breakup and jamming will not result from peaking hydropower operations in the winter, is a major constraint to the production of hydro-electric energy, at a time when it is in high demand. River ice breakup is an extremely complex event, defined by the continuous interaction of time- and space-dependent factors, such as weather conditions, river flows, channel bathymetry and plan geometry, river slopes, ice strength, thickness, and fracture behaviour, as well as thermodynamic processes. It is only recently that the breakup has began to be studied in a quantitative manner and methods to predict some of its more important features have been developed. Though there is still much to be learned, it is useful to summarize the current knowledge, presently scattered in various journals, conference proceedings, and limited-distribution reports. This report describes the various phases of breakup, including the conditioning phase during which the ice cover loses strength and is fractured by hydrodynamic and gravitational forces; the onset phase when parts of the ice fractured ice cover are set in motion and break down into a multitude of blocks; the jamming phase, characterized by the presence of ice jams that form where the ice blocks accumulate against segments of still intact ice cover; and the ice run which is the result of ice jam release and concomitant surges.