This dissertation is organized into three major areas:​ first, healing and normal ligament mechanical properties and in vitro biological adaptation in response to mechanical loading;​ second, structural and microstructural mathematical modeling of tendon and ligament mechanical behavior;​ and third, electron and confocal microscopic investigations of healing and normal ligament and tendon structural behaviors. In the first section an in vitro model for mechanical loading of healing rat medial collateral ligaments (MCLs) is developed. Collagen synthesis in response to mechanical loading is measured using ³H-proline as a metabolic marker. Baseline mechanical properties of healing rat MCL scar and retracted region tissue are established. In the second section, a structural and microstructural mathematical model for tendon and ligament mechanical behavior is proposed and illustrated. The model assumes a Weibull probability distribution function for fiber straightening and three dimensional orientation distribution function for fibril orientation. All parameters in the model have a physical basis. The model quantifies low-load behavior (LLB) as well as the linear region, damage accumulation, and eventual failure of the tissue. It is used to correlate ligament LLB to traditional measures of joint-ligament laxity in a rabbit MCL model and to establish an objective preload independent method of determining experimental reference length. In the third section, scanning electron microscopy is used to investigate collagen matrix fiber structure in mechanically loaded, slack, and reference configuration healing and normal ligaments. Collagen fibers in scar tissue, while not as well organized and oriented as in normal tissue, exhibit waviness in a slack configuration which is removed upon loading and elongation of the tissue. Retracted region tissue shows a greater degree of waviness and more numerous randomly oriented fibers than normal tissue. Confocal microscopy is used to compare strain measured using surface markers on tendon specimens and strain measured locally in the collagen fiber matrix using cells as markers. Results show that both are of the same magnitude with large variability observed in local matrix strain. In summary, this dissertation establishes analytical and experimental techniques to better understand tendon and ligament collagen fiber matrix mechanics and biological adaptation in response to mechanical loading.