Collagens are the most abundant proteins in mammals, comprising about 30% of total protein mass. They are characterized by a triple-helical domain and are deposited in the extracellular matrix, where they form supramolecular assemblies. Four collagens are type II membrane proteins that can also exist in a soluble form. Collagens play structural roles and contribute to the mechanical properties, organization, and shape of tissues. They interact with cells via several receptor families and regulate cell proliferation, migration, and differentiation. Some collagens have restricted tissue distributions and specific biological functions. The collagen superfamily includes 28 members, numbered I–XXVIII in vertebrates. The common structural feature of collagens is the presence of a triple helix, which can range from 96% to less than 10% of the molecule. The diversity of the collagen family is increased by the existence of several α chains, molecular isoforms, and supramolecular structures for a single collagen type. The use of alternative promoters and alternative splicing contributes to the existence of several isoforms. Collagens consist of three α chains, numbered with Arabic numerals. Beyond the 28 collagen types, further diversity occurs due to the existence of several molecular isoforms for the same collagen type and hybrid isoforms composed of α chains from different collagen types. The use of two alternative promoters gives different forms of α1(IX) and α(XVIII) chains, and alternative splicing contributes to the existence of several isoforms of various α chains. The structural organization of collagens includes a triple helix stabilized by glycine, proline, and hydroxyproline residues, interchain hydrogen bonds, and electrostatic interactions. Collagens are multidomain proteins with non-collagenous domains that participate in structural assembly and confer biological activities. These domains are frequently repeated within the same collagen molecule and are also found in other extracellular proteins. Collagens form various supramolecular assemblies, including fibrils, beaded filaments, anchoring fibrils, and networks. Fibril-forming collagens are subdivided into subfamilies based on their supramolecular assemblies. Collagen fibrils are made of various collagen types and can be considered as macromolecular alloys of collagens and non-collagenous proteins or proteoglycans. Collagen fibrillogenesis is regulated by small leucine-rich proteoglycans, collagens V and XIV, and could also influence collagen cross-linking. Collagen biosynthesis involves the synthesis of procollagen molecules, which undergo posttranslational modifications. The stabilization of the procollagen triple helix requires the binding of HSP47. The cleavage of propeptides during maturation is catalyzed by various proteases. The telopeptides contain sites where cross-linking occurs, and this process is initiated by the oxidative deamination of lysyl and hydroxylCollagens are the most abundant proteins in mammals, comprising about 30% of total protein mass. They are characterized by a triple-helical domain and are deposited in the extracellular matrix, where they form supramolecular assemblies. Four collagens are type II membrane proteins that can also exist in a soluble form. Collagens play structural roles and contribute to the mechanical properties, organization, and shape of tissues. They interact with cells via several receptor families and regulate cell proliferation, migration, and differentiation. Some collagens have restricted tissue distributions and specific biological functions. The collagen superfamily includes 28 members, numbered I–XXVIII in vertebrates. The common structural feature of collagens is the presence of a triple helix, which can range from 96% to less than 10% of the molecule. The diversity of the collagen family is increased by the existence of several α chains, molecular isoforms, and supramolecular structures for a single collagen type. The use of alternative promoters and alternative splicing contributes to the existence of several isoforms. Collagens consist of three α chains, numbered with Arabic numerals. Beyond the 28 collagen types, further diversity occurs due to the existence of several molecular isoforms for the same collagen type and hybrid isoforms composed of α chains from different collagen types. The use of two alternative promoters gives different forms of α1(IX) and α(XVIII) chains, and alternative splicing contributes to the existence of several isoforms of various α chains. The structural organization of collagens includes a triple helix stabilized by glycine, proline, and hydroxyproline residues, interchain hydrogen bonds, and electrostatic interactions. Collagens are multidomain proteins with non-collagenous domains that participate in structural assembly and confer biological activities. These domains are frequently repeated within the same collagen molecule and are also found in other extracellular proteins. Collagens form various supramolecular assemblies, including fibrils, beaded filaments, anchoring fibrils, and networks. Fibril-forming collagens are subdivided into subfamilies based on their supramolecular assemblies. Collagen fibrils are made of various collagen types and can be considered as macromolecular alloys of collagens and non-collagenous proteins or proteoglycans. Collagen fibrillogenesis is regulated by small leucine-rich proteoglycans, collagens V and XIV, and could also influence collagen cross-linking. Collagen biosynthesis involves the synthesis of procollagen molecules, which undergo posttranslational modifications. The stabilization of the procollagen triple helix requires the binding of HSP47. The cleavage of propeptides during maturation is catalyzed by various proteases. The telopeptides contain sites where cross-linking occurs, and this process is initiated by the oxidative deamination of lysyl and hydroxyl