Tendons are a soft tissue composed primarily of type I collagen, which is organized into a hierarchical structure to transmit large forces between muscle and bone. Tendon injuries are common and costly, with treatments remaining ineffective. To develop effective treatments more work must be done to understand the processes underlying tendon health. Besides collagens, proteoglycans are a class of proteins that compose the tendon extracellular matrix (ECM). Decorin and biglycan are abundant in the ECM, two class I small leucine-rich proteoglycans that are capable of binding to collagen I fibrils and regulating fibrillogenesis, the primary process responsible for growth and maturity of the tendon ECM. While previous studies have demonstrated the importance of decorin and biglycan in the formation and maintenance of tendon biomechanical properties, inducible knockdown models have been developed that allow for precise temporal control over expression at the desired experimental timepoint. Using these models this work aims to define the regulatory roles of decorin and biglycan throughout multiple stages of life: during neonatal development, in skeletally mature adult, and throughout the aging process. Additionally, the roles of these proteins in the neonatal healing model will also be determined. A sophisticated viscoelastic biomechanical testing protocol will be used alongside structural analyses to determine the properties of tendon throughout these studies. First, we demonstrated in mature animals that biglycan plays a major role in maintaining tendon properties, a previously unknown role for this protein. Next, we determined that knockdown of decorin in mature animals was beneficial for the aging process, producing tendons with higher mechanical properties that resisted the age-related decline in tendon mechanics. Finally, decorin and biglycan expression was reduced during neonatal development and after injury of the neonate. In both cases, the tendons had a large population of small diameter collagen fibrils with reduced mechanical properties due to the tendon collagen microstructure. In conclusion, this work demonstrated the importance of decorin and biglycan in the regulation and maintenance of tendon mechanical properties and health throughout our lifetimes.