Objective: The objective of this work was to characterize the nonlinear anisotropic material constitutive response of the interspinous ligament (ISL). Methods: Cadaveric test samples of the interspinous ligament were tested using the anisotropic small punch test. The measured force-displacement response served as experimental input into a system identification optimization routine to determine the constitutive material parameters that replicated the measured material response. Results: The constitutive behavior of the ISL is notably different from that reported for knee, shoulder, and hip ligaments. Specifically, the high collagen fiber content and unique collagen architecture provided a stiffer material response. The results from the present work were compared with available data from the literature for the ISL and were found to be consistent with reported failure stress and strain to failure. Conclusion: The ISL has unique constitutive properties and architecture that provide mechanical and clinical stability to the lumbar spine during flexion. The characterization data obtained during accomplishment of this thesis provide valuable insights into these roles. The present work provides a first step to fully characterize and understand both physiological and pathological motion of the spine. Further research is necessary to clarify the contributions of spinal ligaments to spine stability and how damage to spinal ligaments may contribute to chronic lower back pain.