The basic requirements for frost heave are: water, freezing temperatures, freezing point depression, unfrozen films between soil particles and ice, and frost-susceptible soil. Freezing point depression arises for pore size, pressure and osmotic reasons. Pore size and ice pressure determine the temperature at which ice can grow within pore necks. Unfrozen water films between soil particles and pore ice are maintained by osmotic forces of counter-ions satisfying particle charge and dissolved salts excluded during ice formation. The osmotic potential at ice/water interfaces is controlled by the local temperatures, and is greatest at the coldest location on an ice lens base, namely the most inaccessible location above individual soil particles. This osmotic potential above the particles drives re-supply of water to the unfrozen films as new ice is formed on the ice lens base, thereby lifting the overlying ice lens and overlying soil. The ice fingers of the frozen fringe are pulled upward by their attachment to the ice lens, and water potential differences supply water upward through unfrozen films in the frozen fringe from the underlying unfrozen soil. If re-supply of water becomes inadequate to meet heat flow requirements, the lens base and the frozen fringe become colder, the freezing front advances and at some stage a new ice lens is initiated deeper in the soil. The maximum heaving pressure is determined by the temperature at the base of the coldest growing ice lens.