The shear strength of dimension lumber is studied considering both the case of straight-grained, clear material and that where the strength is controlled by the rapid propagation of an existing crack such as an end split. Shear strength in the case of clear material is studied by using a size effect model based on a weakest link approach to fracture. In the case of end splits, the strength of the member is studied by using concepts of linear elastic fracture mechanics. Methods are given for developing allowable stresses in both cases. Considering uniformly distributed loads on which current span tables for design are based, maximum tolerated lengths for end splits are derived.