Ski touring is a form of backcountry skiing (or off-piste skiing) involving traveling over the winter landscape on skis under human power rather than through the assistance of ski lifts or snow vehicles. It can take place in terrain ranging from perfectly flat to extremely steep. Since, unlike alpine skiing, backcountry skiing involves extended stretches over flat and uphill terrain, the skiers' heel must be free to allow a natural walking motion while traversing and ascending. Ski touring has parallels with hiking, backpacking, and mountaineering.