Collagen fibrils, which are large, multicomponent protein assemblies, are the principal source of mechanical strength in animal tissues and can grow to millimetre lengths. The mechanism of fibril formation and the influence of non-collagenous components, particularly proteoglycans, in the hierarchical assembly and organisation of collagen fibrils have yet to be fully determined. In this study, quantitative mass mapping by scanning transmission electron microscopy of entire tendon fibrils showed that some fibrils exhibited thinned regions. The axial mass distributions (AMDs) corresponding to the thinned regions were smooth and linear troughs in mass, consistent with the fusion of two fibril tips. In addition, fusion of entire fibrils isolated from tissue has been induced in vitro for the first time. Tissue fibrils fused at their tips in buffered solution to form either fibril rings (having no ends) or elongated fibrils with thinned regions, which lends support to the hypothesis that fusion of fibril tips can occur and that fusion is a mechanism of fibril elongation during the organisation of the extracellular matrix of tendon. Negative staining showed that fusion occurred between the N-tip of a unipolar or bipolar fibril and the C-tip of a unipolar fibril. Fusion between two N-tips was not observed. In one instance fusion was seen between the C-ends of two unipolar fibrils, indicating that fusion between fibril tips must involve the C-end of a unipolar fibril. Cuprolinic blue staining of intact fibrils localised surface-bound proteoglycans predominantly to the shaft region, with markedly depleted levels at the tips. After enzymic removal of proteoglycans, fibrils aggregated laterally in solution. It is likely that a threshold surface concentration of proteoglycans on the fibril shaft prevents lateral aggregation of fibrils whilst being permissive of fibril elongation by tip-to-tip fusion in vivo. Immunoprecipitation experiments with cultured skin fibroblasts using anti-decorin antibodies and anti-collagen antibodies demonstrated the presence of procollagen-decorin complexes in the cell media, suggesting that decorin (a small proteoglycan found in collagen-rich tissues) binds procollagen prior to fibril assembly. This thesis shows that a relationship exists between collagen and proteoglycans throughout the hierarchical assembly of collagen fibrils. An early step in the formation of fibrils appears to be the assembly of procollagen-proteoglycan complexes, which subsequently assemble into well-defined early fibrils. The early fibrils fuse at their tips to form elongated fibrils and the fusion is regulated by proteoglycans located on the fibril shafts.