Operational experience in Canada during the early seventies showed that hovercraft can make very effective icebreakers. In a low speed mode, the hovercraft causes an air cavity to form under the sheet as it rides onto it. This causes a section of the sheet to become unsupported and fail in bending under the action of its own weight. In a high speed mode, the motion of the craft over the sheet sets up large amplitude flexural gravity waves. Research into both modes of operation has been going on at Memorial University of Newfoundland (MUN) since the mid-eighties. This paper summarizes this work. For the low speed case, scaling laws for the resistance to forward motion of two new geometries were developed and confirmed with data obtained in the ice tank at the Institute for Marine Dynamics (IMD) in Newfoundland. For the high speed case, measurements of sheet deflections in the MUN wave tank and the IMD ice tank showed that a critical speed exists for motion over a sheet. At this speed, sheet deflections are limited only by dissipation and nonlinearities. We believe this critical speed is the source of high speed mode hovercraft icebreaking.Key words: hovercraft, icebreakers, air cavity mode, wave mode.