The force-length relations of a set of linear polyethylene fractions and polymers having most probable molecular weight distributions, encompassing a wide molecular weight range, have been investigated. The study of these polymers can serve as models for semicrystalline polymers. The polymers were crystallized in such a manner as to develop as wide a range as possible in the values of the independent structural variables that describe the crystalline state. Several important generalizations can be made from this work. The sharpness of the transition from a brittle to ductile type deformation is established, as is its dependence on molecular weight and certain of the other key structural parameters. It is also quite evident that there is no unique force-length curve for ductile deformations. The character of these curves is very dependent on molecular weight. The deformation process cannot be described in terms of changes in crystallographic features. Neither is there any influence of the supermolecular structure. The ultimate properties are found to depend only on the weight-average molecular weight, indicating the importance of the noncrystalline regions. The initial modulus also depends on the noncrystalline region, albeit in a complex manner. The main variables involved here are the crystallinity level and the interlamellar thickness. In contrast, the yield stress depends on the structure of the crystallite and associated regions. Possible mechanisms for yielding are discussed. Different portions of the stress-strain curves are governed by different structural and molecular features, indicating the complexity of the process.