The low friction in skiing on snow is due to water films generated through frictional heating. There is, however, uncertainty about the thickness and the distribution of these water films. Since direct observation of the water films is difficult, other methods are applied: large-scale tribometer measurements including temperature measurements, contact area measurements between ski and snow, and numerical modeling of snow and ice friction. The numerical model is verified by comparison with the experimentally determined friction coefficients and slider temperatures.The main conclusions are: sliding on snow and ice can be explained by hydrodynamic principles. Unevenly distributed thin water films are responsible for the low friction observed. The most critical parameter determining friction between skis and snow or ice is the real contact area. Ski friction can be optimized by adjusting the size and the topography of the ski base.