The physics of ice sheet dynamics are presented, with an emphasis on the recently developed understanding of the marine ice sheet instability. The relationship between the shallow approximations to the mechanical equations and the Stokes equations is considered, and the approximations for ice sheets, ice shelves and ice streams presented. The heat transport equations and thermal mechanisms for ice stream generation are outlined. The reverse slope grounding line instability is treated in some detail, and supporting evidence from a calving front velocity formula presented. Hard bed sliding with cavitation and soft bed sliding are covered, and the Röthlisberger channel/linked cavity system cooperation/competition are discussed.