Avalanches are always caused by an external stress on the snow pack; they are not random or spontaneous events. Natural triggers of avalanches include additional precipitation, radiative and convective heating, rock fall, ice fall, and other sudden impacts; however, even a snow pack held at a constant temperature, pressure, and humidity will evolve over time and develop stresses, often from the downslope creep of the snow pack. Human triggers of avalanches include skiers, snowmobiles, and controlled explosive work. The triggering stress load can be either localized to the failure point, or remote. Localized triggers of avalanches are typified by point releases from solar heated rocks. Remotely triggered avalanches occur when a tensile stress wave is transmitted through the slab to the start zone, once the stress wave reaches the start zone a fracture initiates and propagates the failure. Of exceptional note is that avalanches can not only entrain additional snow within the failing slab, but can also, given the sufficient accumulation of overburden due to a smaller avalanche, step down and trigger deeper slab instabilities that would be more resilient against smaller stresses. The triggering of avalanches is an example of critical phenomenon.