Tendon structural and mechanical integrity is essential for overall joint function. Tendon structure is highly aligned and hierarchical, where collagen assembles into fibrils, fibril bundles form fibers, and fibers shaped by tendon cells organize into fascicles. Establishment of tendon structure-function is regulated by coordinated processes between cells and the tendon extracellular matrix. However, the cell-cell and cell-matrix interactions necessary for re-creating the native tendon structure following injury is a significant gap in knowledge and a clinical challenge. Collagen XII is a fibril-associated collagen with interrupted triple helices that associates with the surface of fibril-forming collagens. Collagen XII has an important structural role in modulating collagen fibrillogenesis, interacting with other matrix proteins, and forming flexible bridges between fibrils. And more recently, collagen XII has been shown to regulate cell structure and organization. Clinically, mutations in the Col12a1 gene result in myopathic Ehlers-Danlos syndrome, where symptoms include muscle weakness along with joint hypermobility and contractures. Altogether, this suggests that collagen XII is key matrix protein in the coordinated cell- and matrix-mediated processes necessary for proper tendon formation. Therefore, the overall objective of this dissertation was to define the temporal roles of collagen XII in regulating cell arrangement (Aim 1) and matrix assembly (Aim 2) during tendon development. In Aim 1, to specifically isolate the role of collagen XII in tendons, tendon-targeted scleraxis-Cre collagen XII knockout mice were evaluated for patellar tendon morphology, cell organization, matrix structure and function, and gene expression. We found profound changes in cell and matrix organization at postnatal day 0 along with disrupted matrix organization and structure at later postnatal ages. Interestingly, there were also striking changes histologically and functionally at the tendon insertion site, where the collagen XII knockout appeared to inhibit formation of the tibial tuberosity. In Aim 2, when collagen XII knockdown was induced at postnatal day 0, the effects on matrix organization and structure were minimal. Together, our findings indicate a more substantial role of collagen XII for regulating cell arrangement rather than matrix assembly in the establishment of tendon structure-function.