Ligaments and tendons are fundamental structures in the musculoskeletal systems of vertebrates. Ligaments connect bone to bone to provide stability and allow joints to function correctly, whereas tendons connect bone to muscle to allow the transfer of forces generated by muscles to the skeleton. The wide variety of roles played by different ligaments and tendons requires them to have considerably different mechanical responses to applied forces; however, it is not currently known what gives different ligaments and tendons their differing mechanical properties. In this talk I will derive a new constitutive law for modelling ligaments and tendons within a non-linear elastic framework that is based on their microstructure. I show that not only can all of the parameters of this model be directly experimentally measured, but that the new model also shows better agreement with experimental data than the most commonly used model of soft tissue in the literature.