Large snow avalanches start as slab avalanches. A necessary condition for slab fracture is the existence of a continuous thin weak layer that extends over several hundreds m². To start a shear fracture that propagates along the weak layer, critical strain rates and strains have to be attained locally. Critical values for the start of ductile and eventually brittle fracture propagation are achievable only at the border of superweak inclusions that form by initial fracturing within the weak layer. Initial fracturing depends on snow stratigraphy, loading and local disturbances to the stress and strain fields. Important parameters to be determined are therefore fracture stress, strain rate and strain for typical weak layers, size of initial fractures, stress and strain distributions in snow covers and conditions for the formation of weak layers.