Structurally based constitutive models attempt to exploit the tissue composition and structure to avoid ambiguities in material characterization, and offer insight into the function, structure, and mechanics of tissue components. In the structural approach, the tissues total strain energy is assumed to be the sum of the individual fiber strain energies [1–3]. Although theoretically complete, the fiber architecture must be estimated from assumed distributions and “backed-out” from the mechanical data. Thus, fall realization of the structural approach is significantly limited without direct quantitative structural information to either validate structural model predictions or for direct implementation into the model. This limitation of the structural constitutive modeling approach motivated the development of the following structural constitutive model for planar collagenous tissue that directly incorporates the experimentally measured angular distribution of collagen fibers.