Publisher Summary Bone formation is a complex and tightly regulated genetic program that involves two distinct pathways at different anatomical locations. This chapter deals with fibrillar collagens focusing on collagen type I, the most abundant extracellular protein, especially in bone, where it is essential for bone strength. Fibril-forming collagens are synthesized in precursor form, procollagens. For the majority of cells, procollagen is secreted in the extracellular space, where a specific procollagen aminopeptidase (ADAMTS 2) and a specific procollagen carboxypeptidase (BMP1) cleave the propeptides, thereby triggering spontaneous self-assembly of collagen molecules into fibrils, giving rise to mature collagen molecules. Cleavage of the propeptide decreases the solubility of collagen molecules dramatically. Thus a major extracellular function of C propeptides is thought to prevent fibril formation, while N propeptide influence fibril shape and diameter. Following this, the study discusses the structure and biosynthesis of type I collagen and associated proteins that maintain its homeostasis and recent results into the organization of regulatory elements in type I collagen genes, many of which are based on studies in transgenic mice. Finally, it addresses how collagen synthesis is regulated by cytokines and growth factors.