A transversely isotropic hypo-elastic biphasic model is presented to describe the mechanical behavior of articular cartilage under impact loading. This model allowed the consideration that the Young s modulus in the plane of the surface (plane of isotropy) is higher than that in the direction of loading, which is consistent with the microstructural organization of the tissue. Unlike the isotropic model, the transversely isotropic model was capable of fitting high peak loads observed during the initial stage of rapid indentation on the tissue. This made it possible to devise a method of estimating the infinitesimal material properties from small strain indentation relaxation experiments. Infinitesimal analyses using finite element models showed significant differences in the stress distributions between transversely isotropic and isotropic models for idealized contact problems. Stress distributions in transversely isotropic models were more consistent with surface injuries observed during impact experiments. A hypo-elastic model for the solid skeleton was used to model the nonlinear behavior of cartilage under large deformations. Using this model, the nonlinear elastic properties of rabbit retropatellar cartilage were determined from indentation, relaxation experiments. Experimental and theoretical results revealed the important role of interstitial fluid in supporting compressive stress during rapid impact loading with large deformations of retropatellar cartilage. A significant strain-stiffening effect was observed for rapid loading, limiting tissue strains during blunt insult to the joint. The hypo-elastic properties were used to obtain shear stresses in a plane strain model of the rabbit patella under impact loading. Peak shear stresses on the surface were consistent with surface injuries observed in our rabbit model of post-traumatic osteoarthrosis.